- Email: [email protected]
Evaluation of Iowa's graduated driver's licensing program
Accident Analysis and Prevention 40 (2008) 1401–1405
Contents lists available at ScienceDirect
Accident Analysis and Prevention journal homepage: www.elsevier.com/locate/aap
Evaluation of Iowa’s graduated driver’s licensing program Shauna L. Hallmark a,b,∗ , David A. Veneziano b,1 , Scott Falb c,2 , Michael Pawlovich c,2 , Deborah Witt b,1 a
Department of Civil, Construction, and Environmental Engineering, Iowa State University, United States Center for Transportation Research and Education, 2711 S. Loop Drive, Suite 4700, Ames, IA 50010, United States c Iowa Department of Transportation, 800 Lincoln Way, Ames, IA 50010, United States b
a r t i c l e
i n f o
Article history: Received 12 June 2007 Received in revised form 28 February 2008 Accepted 12 March 2008 Keywords: Graduated driver’s licensing Younger drivers Crash risk Minor school license
a b s t r a c t The effectiveness of Iowa’s graduated driver’s licensing (GDL) program was evaluated for a 4-year period before and after implementation in 1999. Since some changes had occurred in the crash reporting format, changes in crash rates for younger drivers were compared to those for 35–44-year-old drivers (middle-age group of drivers) who were used as a control group. After implementation of GDL, the 14-, 16- and 17-year-old age groups experienced a greater decrease in crash rate than the middle-age control group while 15-year-old experienced a smaller decrease. This suggests that the crash rate for 15-year-old drivers may actually have increased when downward trends were adjusted for. Iowa’s GDL program allows holders of the instruction permit to travel unaccompanied to and from school and school-endorsed activities after obtaining a minor school license. Fifteen-year-old with minor school licenses account for up to 26.7% of 15-year-old license holders yet represent up to 74.8% of 15-year-old drivers involved in crashes (depending on the year) from 1998 to 2004. As a result, 15-year-old drivers with minor school licenses are involved in 7.2–8.9 times more crashes, are 7.7 times more likely to have one or more sanctions, and are 4.8 times more likely to receive one or more moving convictions than their peers with a regular instruction permit. This help may explain why 15-year-old drivers did not seem to benefit from implementation of the GDL program in Iowa. © 2008 Elsevier Ltd. All rights reserved.
1. Background Motor vehicle crashes are the leading cause of death for 15–20year-old in the United States, representing about one-third of all fatalities in this age group. 15–20-year-old also make up only 6.3% of the driving population but are involved in 12.6% of all traffic fatalities (NHTSA, 2005). Crash rates are dramatically higher for younger drivers than for older drivers. In the United States in 1995, the crash rate per million miles driven for 16–19-year-old was more than double the rate for drivers in their 1920s and more than four times the rate for drivers ages 30–69. In order to combat the number and severity of crashes among teenage drivers, as of 2007 all states and the District of Columbia have passed legislation for GDL programs (IIHS, 2007). A graduated licensing process allows younger drivers to learn the driving pro-
∗ Corresponding author at: Center for Transportation Research and Education, 2711 S. Loop Drive, Suite 4700, Ames, IA 50010, United States. Tel.: +1 515 294 5249; fax: +1 515 294 0467. E-mail address: [email protected] (S.L. Hallmark). 1 Tel.: +1 515 294 5249; fax: +1 515 294 0467. 2 Tel.: +1 515 239 1101; fax: +1 515 239 1639. 0001-4575/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.aap.2008.03.002
cess in stages. Most programs have a three-stage approach which consists of a “learner” stage requiring supervised driving at all times, an “intermediate” stage which allows unsupervised driving with restrictions and conditions, and then full licensure. Most states allow teenagers to obtain their “learner” permit between ages 15.5 and 16 (Baker et al., 2007). The impact of GDL programs has generally been positive. In 1983, California implemented a GDL program and evaluated its effect on 16- and 17-year-old. Teens could get a learner’s permit at age 15, a provisional license at age 16, and a full adult license at age 17. Both driver education and driver training were required, and parents had to certify that the student had the required hours of driving practice (NHTSA, 2004). California’s GDL became a law rather than a program in July of 1998. The California GDL program has a stringent passenger restriction (no passengers under age 20 for the first 6 months), and a 1-year nighttime driving restriction (midnight to 5:00 a.m.). Five years after the initiation of California’s GDL program, a report issued by the California Department of Motor Vehicles found that GDL contributed to a 5.3% reduction in the rate of crashes involving drivers ages 15–17 (NHTSA, 2004). The GDL program in Florida was instituted in 1996 for drivers younger than 18. Drivers holding learner’s licenses are restricted from driving between 7:00 p.m. and 6:00 a.m. Learner’s licenses
1402
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
are held for 6 months before drivers are eligible for an intermediate license. Nighttime driving restrictions for 16-year-old with intermediate licenses are from 11:00 p.m. to 6:00 a.m. and for 17year-old from 1:00 a.m. to 6:00 a.m. Drivers younger than 18 have a limit on the number of violations they can accumulate; and drivers younger than 21 are subject to a zero tolerance law for drinking and driving. Crash data for Florida teenage drivers from 1995 to 1997 was compared with crash data from Alabama, which did not have a GDL program in place at the time. A 9% reduction in fatal and injury crashes resulted for the 15–17-year-old age group after implementation of GDL. No reduction was noted for 18-year-old drivers (Ulmer et al., 2000). Mayhew et al. (2003) examined month-to-month changes in collisions among new drivers in Nova Scotia prior to the enactment of a GDL program. They calculated crash rate per driver and evaluated novices (drivers from 6 months to 24 months from first licensing), in age groups 16–19 and 20 and older. A regression model indicated that among novice drivers the crash rate for younger novice drivers was significantly higher during the first few months of driving alone than the older age group. Mayhew et al. also found that both age groups had a reduction in crash rate as experience increased. Both groups experienced a significant decline in crashes during the first 7 months after licensing; although the effect was more significant for younger, rather than older drivers. Baker et al. (2007) recently conducted a nationwide review of GDL programs. They evaluated the effectiveness of programs based on the number of and types of restrictions present in the GDL program using an analysis of fatal and injury crashes. Forty-three states were evaluated using fatalities and 35 states were evaluated for injury crashes. Baker et al. evaluated the period from 1994 to 2004 in quarters (3-month periods), in terms of whether a GDL was in place for the quarter. They used a population-based exposure estimate and considered whether programs had the following components: • Minimum age of 16 years for learner’s permit. • Learner’s permit must be held for at least 6 months before driving unsupervised. • At least 30 h of supervised driving during the learner stage. • Nighttime driving restriction starting at least by 10:00 p.m. • Passenger restriction of no more than one teenage passenger except for family members. • Minimum age of 16.5 years for intermediate license. • Minimum age of 17 years for full licensure. Baker et al. found an 11% lower fatal crash rate for 16-year-old drivers for all programs with a three-stage GDL program, regardless of which specific components were used, as compared to programs that did not have a three-stage GDL program. They also found a 21% lower fatal crash involvement rate for 16-year-old drivers when a GDL included four of the above components and a 38% lower crash rate when five GDL components were in effect. Baker et al. also found that when fewer than four components were present there was no statistically significant reduction in fatal crashes. Baker et al. also found an injury crash rate for 16-year-old drivers that was 19% lower when any GDL program was in place and a 40% lower injury crash rate when a program had five of the GDL components. Their study suggests that the type of restrictions is a critical component of a GDL program. Shope and Molnar (2003) summarized the results of 17 states who had enacted GDL programs from 1996 to 1999. Overall they found that all states had identified some crash reduction after enacting a GDL. They also indicated that fatal or injury crash involvement for 16-year-old drivers had decreased by 11% for
Florida and 24% for Michigan. Involvement in any type of crash decreased for 16-year-old by 35% in Michigan and 27% in North Carolina. A later study by Shope (2007) summarized more recent results of GDL programs in 14 jurisdictions. Shope found that crash reductions were reported for almost all the studies and that results were somewhat consistent with reductions in crashes from 20% to 40% being reported. 2. Description of Iowa’s GDL program Prior to the enactment of Iowa’s current GDL program, young people were able to get an instruction permit at age 14 by passing the Iowa written driver’s test and providing a signed state parental consent form. In order to get a driver’s license by age 16, a young person had to complete a state approved driver education course (30 classroom hours and 6 h of driving). There were no restrictions on driving times or number of passengers, no provisions for early driver improvement intervention, and no requirements for continued parental supervision of driving. Once the young person got his or her “regular” driver’s license, the only restriction that applied to this young person because of his or her age and new driver status was “zero tolerance” for a 0.02 alcohol level which would result in license revocation. In every other instance, the young person with an Iowa driver’s license was subject to the same driver restrictions as the adult driver. The Iowa GDL program was implemented on 1 January 1999 and has three stages. An instruction permit is available at age 14. Instruction permit holders must be supervised by a licensed adult driver and must complete 20 h of supervised driving with a minimum of 2 h between sunrise and sunset. A teenage driver must hold the instruction permit for a minimum of 6 months and drive accident and conviction free for 6 consecutive months immediately preceding application for an intermediate license. Driver’s education is required during the instruction permit stage and consists of 30 h of classroom time and 6 h of driving. Iowa has a provision that allows instruction permit holders who are 16 years old and younger to obtain a minor school license which allows the licensee to travel unaccompanied to and from the school of attendance and school-endorsed activities by the most direct route. The school license is available after the applicant has held an instruction permit for 6 months and has been crash and conviction free for that period. However, minor school license holders are not required to carry any documentation about times when endorsed school activities are occurring or a map showing their home and the shortest location to the school. As a result, an officer who stops a solo minor school license holder has no way of verifying whether this minor is complying with the terms of the minor school license or is driving alone to other activities. An intermediate license is available at age 16 and must be held for 12 months. A nighttime restriction requires that the teenage driver be supervised by a licensed adult driver between the hours of 12:30 a.m. and 5:00 a.m. Intermediate license holders must complete 10 h of supervised driving with a minimum of 2 h from sunrise to sunset. If a driver is convicted of a moving violation or contributes to a crash, he or she is required to attend a remedial driver interview with his or her parents before a Department of Transportation (DOT) driver’s license representative. As a minimum, the driver is required to retain his or her intermediate license for another year. Additionally, new restrictions may be added to the license by the DOT or the driver may have his or her license downgraded to an instruction permit or have the license suspended. The number of passengers that can ride in a vehicle with a young driver is limited by the number of seat belts in the car for both the instruction permit and intermediate license stages.
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
Full driving privileges without restriction can be obtained if the driver meets the requirements at the age of 17, unless there are violations or crashes with contributable cause. All conditions of the intermediate license must be met and the driver must have written approval of a parent or guardian (Iowa DOT, 2007). At 18 years of age and older, drivers may take the regular driving test and obtain a license without complying with any of the requirements of the GDL. 3. Methodology The effectiveness of Iowa’s GDL program in reducing crashes was evaluated using a 4-year analysis period before and after the GDL was instituted. The Iowa GDL took effect in 1999, so the before period was made up of crash data for 1995–1998. Crashes for 1999 were excluded since not all teenage drivers during 1999 were subject to the GDL program. The Iowa DOT crash reporting form changed in 2001 and due to problems during the transition, data quality was uncertain. Consequently, crash data for 2001 were not used in the analysis. Crash data were provided by the Iowa DOT. Crash data were analyzed for drivers in each group from 14-yearold to 17-year-old. Drivers in the 14–16-year-old age groups would have been uniformly subject to the new GDL restrictions from 2000 and later. As a result, 2000, 2002–2004 were used as the “after” period for those age groups. Drivers who turned 16 in 1999 were not uniformly affected by GDL. Some may have fallen under the new restrictions and others may not have. Consequently, the after analysis period for 17-year-old included 2002–2004. These periods would have included 17-year-old who were uniformly subject to the new GDL requirements. The crash form and reporting requirements for crashes in Iowa changed in 2001. As stated, this year was removed from the analysis since data quality was uncertain. The change also affected the way crashes are reported so crashes from the before period are not completely consistent with crashes for the after period. One change with the 2001 crash form is that drivers are no longer required to file a personal crash report if a police officer investigates the crash and files a report. As a result, if the officer does not file the report or the crash report is mislaid, there is no longer an additional record of the occurrence. Additionally, some police agencies changed internal reporting policies which resulted in a reduction in the reporting of some crashes that resulted in property damage. Both factors resulted in fewer reported crashes starting in 2001. As a result, the before and after periods were not entirely consistent. Additionally, in any before and after study to evaluate the effect of a treatment, there is always the potential that factors other than the treatment may have been responsible for a reduction in crashes. As a result, direct comparison of the crash rate before and after implementation of GDL was not appropriate. In order to determine the effect of implementing GDL versus independent changes due to differences in crash reporting or other trends, a method suggested by Hauer (1997) was used. Hauer suggests comparing crash reduction for the treatment group to crash reduction in a control group which was not likely to have been
1403
affected by the treatment. This control population is used to control factors outside of the treatment that affect crashes, which cannot be accounted for but may affect study results. The middle-age group, 35–44-year-old, was used as the control population for this study, based on an approach used in other studies. Shope and Molnar (2004) compared the crash risk for 16year-old in Michigan to those of drivers age of 25 years and older who were not expected to be affected by GDL. Ulmer et al. (2000) used drivers ages 25–54 as a control group. Middle-age drivers represent a stable group of drivers and were used to account for the downward crash trend due to changes in the crash reporting system and other overall crash trends which were independent of the GDL. Crash rate was compared for each age group during the before and after periods using: CRti =
Cti Nti
(1)
where CRti is the crash rate per licensed driver for age group i for time period t, Cti is the number of crashes for age group i for time period t, and Nti is the number of licensed drivers for age group i for time period t. An odds ratio was used to evaluate the impact of the GDL program to account for factors unrelated to GDL using the odds ratio as suggested by Hauer (1997) and Harwood et al. (2002). The odds ratio represents the ratio of the expected number of crashes in the after period to the expected number of accidents in the before period for the treatment group, divided by the ratio of the expected number of crashes in the after period to the expected number of accidents in the before period for the treatment group, as shown in the following equation: odds ratio =
P(A|group 1)/[1 − P(A|group 1)] P(A|group 2)/[1 − P(A|group 2)]
(2)
where A is any event with probabilities P(A|group 1) and P(A|group 2). The 95% confidence intervals were obtained by taking the natural log of the odds ratio ± 1.96 times the standard error (Ott and Longnecker, 2001). 4. Results Crash characteristics are shown in Table 1. As shown, the crash rate for all age groups was lower after implementation of GDL than before. All teenage groups experienced a larger decrease in the crash rate than the 35–44-year-old age group (21.6%), except for 15-year-old who experienced a smaller reduction (15.3%). All decreases were statistically significant at the 95% level of significance using a test of proportionality in the following equation (Ott and Longnecker, 2001): z=
ˆ1 − ˆ2 ˆ 1 (1 − ˆ 1 )/n1 ) + ( ˆ 2 (1 − ˆ 2 )/n2 ) (
(3)
Table 1 Characteristics of crashes Age group
14-year olds 15-year olds 16-year olds 17-year olds 35–44-year olds
Crashes
Crash rate (crashes/licensed driver)
Before
After
Before
After
1,219 3,425 27,477 24,336 91,791
719 2,816 17,740 11,998 66,967
0.017 0.027 0.182 0.154 0.055
0.009 0.023 0.127 0.119 0.043
Decrease (%)
Odds ratio (95% confidence interval)
42.9 15.3 30.2 25.3 21.6
0.81 (0.74, 0.89) 1.13 (1.07, 1.19) 0.89 (0.87, 0.90) 0.68 (0.66, 0.69)
1404
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
Table 2 Crashes by license type for 15-year-old drivers 1996
1997
1998
15-year-old drivers involved in crashes by license type Minor school license 442 (64.2%) Instruction permit 247 (35.8%)
421 (67.9%) 199 (32.1%)
Number of 15-year-old licensed drivers Total licensed drivers 31,990 Minor school license Data not available
31,990 Data not available
Crash rate per 1000 licensed drivers Minor school license N/A Instruction permit N/A
N/A N/A
2001
2002
2003
2004
408 (70.7%) 194 (29.3%)
370 (70.6%) 154 (29.4%)
430 (70.7%) 178 (29.3%)
428 (71.5%) 171 (25.7%)
463 (73.8%) 164 (26.2%)
31,287 6,690 (21.4%)
31,038 7,106 (22.9%)
30,287 7,342 (24.2%)
30,819 7,925 (25.7%)
31,617 8,456 (26.7%)
61.0 6.9
52.1 6.4
58.6 7.8
54.0 7.5
54.8 7.1
8.1
7.5
7.2
7.7
Likelihood of school license holder to be involved in a crash compared to instruction permit holder N/A N/A N/A 8.9 Crash data were not available by license type for 1995
ˆ is the sample proportion (number of where z is the z-statistic, successes divided by sample size (n)), and n is the sample size. The odds ratio for each age group is also shown in Table 1 and Eq. (3). An odds ratio less than one suggest that there are proportionally fewer crashes in the treatment group than the control group. Evaluation of the 14-, 16-, and 17-year-old driver age groups resulted in odds ratios less than one, indicating that GDL was effective in reducing crashes for these age groups. The odds ratio for the 15year-old age group was 1.13, suggesting that when accounting for factors not related to the GDL program, crashes increased. 5. Effect of school license GDL appears to have been successful for 14-, 16-, and 17-yearold drivers who experienced a reduction in crash rate per licensed driver even after adjusting for the decrease in crash rate for middleage drivers, which was used to reflect an overall downward trend in crash rate. The crash rate for 15-year-old drivers decreased at a much smaller rate than the decrease experienced by middle-age drivers from the before to after period suggesting that the crash rate for 15year-old drivers would actually have increased from the before to after period had other downward trends not influencing the number of crashes. One factor that may have impacted the effectiveness of GDL in reducing the crash rate for younger drivers is the influence of school licenses. In Iowa, teenage drivers age 14.5–16 who have instruction permits may obtain a minor school license. This license allows them to drive unaccompanied to and from school or school-sanctioned extracurricular activities. Crashes by license type are shown in Table 2 for 15-year-old drivers by year from 1996 to 1998 for the before period and 2001–2004 for the after analysis period. Information about the type of license held was not available in the 1995 crash data set. The number of 15-year-old licensed drivers and the number of those who hold a minor school license are provided for 1998 and 2001–2004. Only estimated values existed for 1997 and 1996 and no information
was available about the number of minor school licenses held for 1995. As a result, it was not possible to compare the change in crash rate for minor school license holders from the before to after period. As shown, 15-year-old instruction permit holders accounted for 21.4–26.7% of the license holders and made up 70. 5–73.8% of 15-year-old drivers involved in crashes where the license type was known (1998 and 2001–2004 values). As a result, 15-year-old drivers with minor school licenses were involved in 7.2–8.9 times more crashes than 15-year-old operating under a regular instruction permit. Crash rate by license type is also shown in Table 2. There appears to be decreases in the various metrics shown in Table 2 when comparing 1998 values to 2001–2004; although using only one data point for the before period is not conclusive. However, crashes in Iowa decreased overall by about 20% from the before to after period so decreases shown here are consistent with what was occurring in other crash statistics. Information about sanctions and moving violation convictions was also obtained for 14- and 15-year-old drivers. This information was only available from the Iowa Department of Motor Vehicles for the 6-month period from September 2004 to April 2005. As shown in Table 3, 14- and 15-year-old school license holders are 7.7 times more likely to have one or more sanctions than their peers with a regular instruction permit. They are also 4.8 times more likely to receive one or more moving convictions. It should also be noted that teenage drivers with minor school licenses have their minor school licenses suspended after one conviction and revoked after two. As a result, there are few school license holders left after one conviction. School license holders are only likely to have two or more convictions if they committed the convictions in fairly quick succession before their minor school license was suspended or revoked. Regular instruction permit holders who receive convictions attend a remedial driver interview which provides consequences but not necessarily suspension or revocation of their license. The fact that it is difficult for school license holders to get two or more violations and yet their numbers are still similar to those with instruction permits suggests that a school license holder may be racking up violations in short amounts of time.
Table 3 Sanctions and moving violations for 14- and 15-year-old drivers by license type (September 2004–April 2005) License Type
None
1
Number of driver sanctions Instruction permit School license
2
3+
Total
48,799 (99.1%) 8,068 (92.8%)
341 (0.7%) 541 (6.2%)
65 (0.1%) 59 (0.7%)
53 (0.1%) 25 (0.3%)
49,258 8,693
Number of moving violation convictions Instruction permit 48,453 (98.4%) School license 8,009 (92.1%)
524 (1.1%) 545 (6.3%)
189 (0.4%) 121 (1.4%)
92 (0.2%) 18 (0.2%)
49,258 8,693
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
The authors realize that the above calculations do not account for exposure. Teenagers with a minor school license may be more likely to drive more frequently and for longer distances than their counterparts with a regular instruction permit. However, there was no reasonable method for obtaining exposure to account for this difference. The group was too small to adequately use the induced exposure method. It is expected that exposure accounts for some of the increase in crashes and violations for minor school license holders. Regardless, however, the point of a GDL program is to allow for teenage drivers to learn the process of driving in stages under the supervision of a responsible adult. Since many of the trips that a teenager takes are likely to be school and school-activity related, the minor school license allows inexperienced teenagers to negate the purpose of a GDL by allowing them to drive unsupervised. There is also currently no way for an officer who pulls over a teenager who is driving alone under a school license to verify whether the teenager is enroute to school or a school-sanctioned activity. As a result, this group of teenage drivers may be spending a significant time driving alone even without attending school activities. As shown in Table 2, 15-year-old drivers with minor school permits are consistently involved in significantly more crashes than their peers with instruction permits. 6. Summary and conclusions The effectiveness of the Iowa graduated drivers licensing program was evaluated for a 4-year period before and after implementation in 1999. Since some changes had occurred in the crash reporting form and the types of crashes which are required to be reported, changes in crash rates for younger drivers were compared to the reduction in crash rate for 35–44-year-old drivers who are assumed to represent a stable group and were used to reflect general downward trends. After implementation of GDL, the crash rate for all age groups in the after period was lower than in the before period. The teenage groups (14-, 16-, and 17-year-old) experienced a greater decrease in crash rate than the 35–44-year-old age group while the 15-year-old experienced a smaller decrease. This suggests that the crash rate for 15-year-old drivers actually increases when downward trends are adjusted for. The results for the 15-year-old age group were unexpected. The rate of compliance to GDL restrictions for any age group is unknown
1405
so the effect of compliance could not be evaluated. As indicated, those with minor school licenses were far more likely to be involved in a crash as well as have more moving violations and convictions. The prevalence of the minor school license may have been the major factor that resulted in failure of GDL to reduce crashes for 15-yearold. Results suggest that room for improvement exists in the Iowa GDL program. The most obvious would be to reconsider granting minor school licenses. Acknowledgement The research team wishes to thank the Iowa Department of Transportation for funding this project. References Baker, S.P., Chen, L.-H., Li, G., 2007. Nationwide Review of Graduated Driver Licensing. AAA Foundation for Traffic Safety, Washington, DC. Harwood, D.M., Bauer, K.M., Potts, I.B., Torbic, D.J., Richard, K.R., Kohlman Rabbani, E.R., Hauer, E., Elefteriaduou, L., 2002. Safety effectiveness of intersection leftand right-turn lanes. Federal Highway Administration, Office of Safety Research Development. FHWA-RD-02-089. Hauer, E., 1997. Observational Before–After Studies in Road Safety. Pergamon, New York, 1997. Institute for Highway Safety. Licensing System for Young Drivers. http://www. iihs.org/laws/graduatedLicenseIntro.aspx. Accessed November 2007. Iowa Office of Driver Services. Graduated Driver Licenses (GDL). Information on Iowa’s System for Drivers under the Age of 18. http://www.iamvd.com/ods/ gdl.htm. Accessed September 2007. Mayhew, D.R., Simpson, H.M., Pak, A., 2003. Changes in collision rates among novice drivers during the first months of driving. Accident Anal. Prev. 5, 683–691. National Highway Traffic Safety Association. Graduated Driver Licensing System. http://www.nhtsa.dot.gov/people/outreach/safesobr/19qp/factsheets/graduated.html. Accessed January 7, 2004. National Highway Traffic Safety Association, 2005. Traffic Safety Facts: 2005 Data, Young Drivers. Online html Document: http://www-nrd.nhtsa.dot.gov/ Pubs/810630.PDF. Accessed November 2007. Ott, R.L., Longnecker, M., 2001. An Introduction to Statistical Methods and Data Analysis. Duxbury, Thomson Learning, Pacific Grove, CA. Shope, J.T., 2007. Graduated Driver Licensing: Review of Evaluation Results Since 2002. J. Safety Res. 38, 165–175. Shope, J.L., Molnar, L.J., 2003. Graduated driver licensing in the United States: evaluation results from the early programs. J. Safety Res. 34, 63–69. Shope, J.L., Molnar, L.J., 2004. Michigan’s graduated driver licensing program: evaluation of the first four years. J. Safety Res. 35, 337–344. Ulmer, R.G., Preusser, D.F., Williams, A.F., Ferguson, S.A., Farmer, C.M., 2000. Effect of Florida’s graduated driver licensing program on the crash rate of teenage drivers. Accident Anal. Prev. 32, 527–532.
Contents lists available at ScienceDirect
Accident Analysis and Prevention journal homepage: www.elsevier.com/locate/aap
Evaluation of Iowa’s graduated driver’s licensing program Shauna L. Hallmark a,b,∗ , David A. Veneziano b,1 , Scott Falb c,2 , Michael Pawlovich c,2 , Deborah Witt b,1 a
Department of Civil, Construction, and Environmental Engineering, Iowa State University, United States Center for Transportation Research and Education, 2711 S. Loop Drive, Suite 4700, Ames, IA 50010, United States c Iowa Department of Transportation, 800 Lincoln Way, Ames, IA 50010, United States b
a r t i c l e
i n f o
Article history: Received 12 June 2007 Received in revised form 28 February 2008 Accepted 12 March 2008 Keywords: Graduated driver’s licensing Younger drivers Crash risk Minor school license
a b s t r a c t The effectiveness of Iowa’s graduated driver’s licensing (GDL) program was evaluated for a 4-year period before and after implementation in 1999. Since some changes had occurred in the crash reporting format, changes in crash rates for younger drivers were compared to those for 35–44-year-old drivers (middle-age group of drivers) who were used as a control group. After implementation of GDL, the 14-, 16- and 17-year-old age groups experienced a greater decrease in crash rate than the middle-age control group while 15-year-old experienced a smaller decrease. This suggests that the crash rate for 15-year-old drivers may actually have increased when downward trends were adjusted for. Iowa’s GDL program allows holders of the instruction permit to travel unaccompanied to and from school and school-endorsed activities after obtaining a minor school license. Fifteen-year-old with minor school licenses account for up to 26.7% of 15-year-old license holders yet represent up to 74.8% of 15-year-old drivers involved in crashes (depending on the year) from 1998 to 2004. As a result, 15-year-old drivers with minor school licenses are involved in 7.2–8.9 times more crashes, are 7.7 times more likely to have one or more sanctions, and are 4.8 times more likely to receive one or more moving convictions than their peers with a regular instruction permit. This help may explain why 15-year-old drivers did not seem to benefit from implementation of the GDL program in Iowa. © 2008 Elsevier Ltd. All rights reserved.
1. Background Motor vehicle crashes are the leading cause of death for 15–20year-old in the United States, representing about one-third of all fatalities in this age group. 15–20-year-old also make up only 6.3% of the driving population but are involved in 12.6% of all traffic fatalities (NHTSA, 2005). Crash rates are dramatically higher for younger drivers than for older drivers. In the United States in 1995, the crash rate per million miles driven for 16–19-year-old was more than double the rate for drivers in their 1920s and more than four times the rate for drivers ages 30–69. In order to combat the number and severity of crashes among teenage drivers, as of 2007 all states and the District of Columbia have passed legislation for GDL programs (IIHS, 2007). A graduated licensing process allows younger drivers to learn the driving pro-
∗ Corresponding author at: Center for Transportation Research and Education, 2711 S. Loop Drive, Suite 4700, Ames, IA 50010, United States. Tel.: +1 515 294 5249; fax: +1 515 294 0467. E-mail address: [email protected] (S.L. Hallmark). 1 Tel.: +1 515 294 5249; fax: +1 515 294 0467. 2 Tel.: +1 515 239 1101; fax: +1 515 239 1639. 0001-4575/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.aap.2008.03.002
cess in stages. Most programs have a three-stage approach which consists of a “learner” stage requiring supervised driving at all times, an “intermediate” stage which allows unsupervised driving with restrictions and conditions, and then full licensure. Most states allow teenagers to obtain their “learner” permit between ages 15.5 and 16 (Baker et al., 2007). The impact of GDL programs has generally been positive. In 1983, California implemented a GDL program and evaluated its effect on 16- and 17-year-old. Teens could get a learner’s permit at age 15, a provisional license at age 16, and a full adult license at age 17. Both driver education and driver training were required, and parents had to certify that the student had the required hours of driving practice (NHTSA, 2004). California’s GDL became a law rather than a program in July of 1998. The California GDL program has a stringent passenger restriction (no passengers under age 20 for the first 6 months), and a 1-year nighttime driving restriction (midnight to 5:00 a.m.). Five years after the initiation of California’s GDL program, a report issued by the California Department of Motor Vehicles found that GDL contributed to a 5.3% reduction in the rate of crashes involving drivers ages 15–17 (NHTSA, 2004). The GDL program in Florida was instituted in 1996 for drivers younger than 18. Drivers holding learner’s licenses are restricted from driving between 7:00 p.m. and 6:00 a.m. Learner’s licenses
1402
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
are held for 6 months before drivers are eligible for an intermediate license. Nighttime driving restrictions for 16-year-old with intermediate licenses are from 11:00 p.m. to 6:00 a.m. and for 17year-old from 1:00 a.m. to 6:00 a.m. Drivers younger than 18 have a limit on the number of violations they can accumulate; and drivers younger than 21 are subject to a zero tolerance law for drinking and driving. Crash data for Florida teenage drivers from 1995 to 1997 was compared with crash data from Alabama, which did not have a GDL program in place at the time. A 9% reduction in fatal and injury crashes resulted for the 15–17-year-old age group after implementation of GDL. No reduction was noted for 18-year-old drivers (Ulmer et al., 2000). Mayhew et al. (2003) examined month-to-month changes in collisions among new drivers in Nova Scotia prior to the enactment of a GDL program. They calculated crash rate per driver and evaluated novices (drivers from 6 months to 24 months from first licensing), in age groups 16–19 and 20 and older. A regression model indicated that among novice drivers the crash rate for younger novice drivers was significantly higher during the first few months of driving alone than the older age group. Mayhew et al. also found that both age groups had a reduction in crash rate as experience increased. Both groups experienced a significant decline in crashes during the first 7 months after licensing; although the effect was more significant for younger, rather than older drivers. Baker et al. (2007) recently conducted a nationwide review of GDL programs. They evaluated the effectiveness of programs based on the number of and types of restrictions present in the GDL program using an analysis of fatal and injury crashes. Forty-three states were evaluated using fatalities and 35 states were evaluated for injury crashes. Baker et al. evaluated the period from 1994 to 2004 in quarters (3-month periods), in terms of whether a GDL was in place for the quarter. They used a population-based exposure estimate and considered whether programs had the following components: • Minimum age of 16 years for learner’s permit. • Learner’s permit must be held for at least 6 months before driving unsupervised. • At least 30 h of supervised driving during the learner stage. • Nighttime driving restriction starting at least by 10:00 p.m. • Passenger restriction of no more than one teenage passenger except for family members. • Minimum age of 16.5 years for intermediate license. • Minimum age of 17 years for full licensure. Baker et al. found an 11% lower fatal crash rate for 16-year-old drivers for all programs with a three-stage GDL program, regardless of which specific components were used, as compared to programs that did not have a three-stage GDL program. They also found a 21% lower fatal crash involvement rate for 16-year-old drivers when a GDL included four of the above components and a 38% lower crash rate when five GDL components were in effect. Baker et al. also found that when fewer than four components were present there was no statistically significant reduction in fatal crashes. Baker et al. also found an injury crash rate for 16-year-old drivers that was 19% lower when any GDL program was in place and a 40% lower injury crash rate when a program had five of the GDL components. Their study suggests that the type of restrictions is a critical component of a GDL program. Shope and Molnar (2003) summarized the results of 17 states who had enacted GDL programs from 1996 to 1999. Overall they found that all states had identified some crash reduction after enacting a GDL. They also indicated that fatal or injury crash involvement for 16-year-old drivers had decreased by 11% for
Florida and 24% for Michigan. Involvement in any type of crash decreased for 16-year-old by 35% in Michigan and 27% in North Carolina. A later study by Shope (2007) summarized more recent results of GDL programs in 14 jurisdictions. Shope found that crash reductions were reported for almost all the studies and that results were somewhat consistent with reductions in crashes from 20% to 40% being reported. 2. Description of Iowa’s GDL program Prior to the enactment of Iowa’s current GDL program, young people were able to get an instruction permit at age 14 by passing the Iowa written driver’s test and providing a signed state parental consent form. In order to get a driver’s license by age 16, a young person had to complete a state approved driver education course (30 classroom hours and 6 h of driving). There were no restrictions on driving times or number of passengers, no provisions for early driver improvement intervention, and no requirements for continued parental supervision of driving. Once the young person got his or her “regular” driver’s license, the only restriction that applied to this young person because of his or her age and new driver status was “zero tolerance” for a 0.02 alcohol level which would result in license revocation. In every other instance, the young person with an Iowa driver’s license was subject to the same driver restrictions as the adult driver. The Iowa GDL program was implemented on 1 January 1999 and has three stages. An instruction permit is available at age 14. Instruction permit holders must be supervised by a licensed adult driver and must complete 20 h of supervised driving with a minimum of 2 h between sunrise and sunset. A teenage driver must hold the instruction permit for a minimum of 6 months and drive accident and conviction free for 6 consecutive months immediately preceding application for an intermediate license. Driver’s education is required during the instruction permit stage and consists of 30 h of classroom time and 6 h of driving. Iowa has a provision that allows instruction permit holders who are 16 years old and younger to obtain a minor school license which allows the licensee to travel unaccompanied to and from the school of attendance and school-endorsed activities by the most direct route. The school license is available after the applicant has held an instruction permit for 6 months and has been crash and conviction free for that period. However, minor school license holders are not required to carry any documentation about times when endorsed school activities are occurring or a map showing their home and the shortest location to the school. As a result, an officer who stops a solo minor school license holder has no way of verifying whether this minor is complying with the terms of the minor school license or is driving alone to other activities. An intermediate license is available at age 16 and must be held for 12 months. A nighttime restriction requires that the teenage driver be supervised by a licensed adult driver between the hours of 12:30 a.m. and 5:00 a.m. Intermediate license holders must complete 10 h of supervised driving with a minimum of 2 h from sunrise to sunset. If a driver is convicted of a moving violation or contributes to a crash, he or she is required to attend a remedial driver interview with his or her parents before a Department of Transportation (DOT) driver’s license representative. As a minimum, the driver is required to retain his or her intermediate license for another year. Additionally, new restrictions may be added to the license by the DOT or the driver may have his or her license downgraded to an instruction permit or have the license suspended. The number of passengers that can ride in a vehicle with a young driver is limited by the number of seat belts in the car for both the instruction permit and intermediate license stages.
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
Full driving privileges without restriction can be obtained if the driver meets the requirements at the age of 17, unless there are violations or crashes with contributable cause. All conditions of the intermediate license must be met and the driver must have written approval of a parent or guardian (Iowa DOT, 2007). At 18 years of age and older, drivers may take the regular driving test and obtain a license without complying with any of the requirements of the GDL. 3. Methodology The effectiveness of Iowa’s GDL program in reducing crashes was evaluated using a 4-year analysis period before and after the GDL was instituted. The Iowa GDL took effect in 1999, so the before period was made up of crash data for 1995–1998. Crashes for 1999 were excluded since not all teenage drivers during 1999 were subject to the GDL program. The Iowa DOT crash reporting form changed in 2001 and due to problems during the transition, data quality was uncertain. Consequently, crash data for 2001 were not used in the analysis. Crash data were provided by the Iowa DOT. Crash data were analyzed for drivers in each group from 14-yearold to 17-year-old. Drivers in the 14–16-year-old age groups would have been uniformly subject to the new GDL restrictions from 2000 and later. As a result, 2000, 2002–2004 were used as the “after” period for those age groups. Drivers who turned 16 in 1999 were not uniformly affected by GDL. Some may have fallen under the new restrictions and others may not have. Consequently, the after analysis period for 17-year-old included 2002–2004. These periods would have included 17-year-old who were uniformly subject to the new GDL requirements. The crash form and reporting requirements for crashes in Iowa changed in 2001. As stated, this year was removed from the analysis since data quality was uncertain. The change also affected the way crashes are reported so crashes from the before period are not completely consistent with crashes for the after period. One change with the 2001 crash form is that drivers are no longer required to file a personal crash report if a police officer investigates the crash and files a report. As a result, if the officer does not file the report or the crash report is mislaid, there is no longer an additional record of the occurrence. Additionally, some police agencies changed internal reporting policies which resulted in a reduction in the reporting of some crashes that resulted in property damage. Both factors resulted in fewer reported crashes starting in 2001. As a result, the before and after periods were not entirely consistent. Additionally, in any before and after study to evaluate the effect of a treatment, there is always the potential that factors other than the treatment may have been responsible for a reduction in crashes. As a result, direct comparison of the crash rate before and after implementation of GDL was not appropriate. In order to determine the effect of implementing GDL versus independent changes due to differences in crash reporting or other trends, a method suggested by Hauer (1997) was used. Hauer suggests comparing crash reduction for the treatment group to crash reduction in a control group which was not likely to have been
1403
affected by the treatment. This control population is used to control factors outside of the treatment that affect crashes, which cannot be accounted for but may affect study results. The middle-age group, 35–44-year-old, was used as the control population for this study, based on an approach used in other studies. Shope and Molnar (2004) compared the crash risk for 16year-old in Michigan to those of drivers age of 25 years and older who were not expected to be affected by GDL. Ulmer et al. (2000) used drivers ages 25–54 as a control group. Middle-age drivers represent a stable group of drivers and were used to account for the downward crash trend due to changes in the crash reporting system and other overall crash trends which were independent of the GDL. Crash rate was compared for each age group during the before and after periods using: CRti =
Cti Nti
(1)
where CRti is the crash rate per licensed driver for age group i for time period t, Cti is the number of crashes for age group i for time period t, and Nti is the number of licensed drivers for age group i for time period t. An odds ratio was used to evaluate the impact of the GDL program to account for factors unrelated to GDL using the odds ratio as suggested by Hauer (1997) and Harwood et al. (2002). The odds ratio represents the ratio of the expected number of crashes in the after period to the expected number of accidents in the before period for the treatment group, divided by the ratio of the expected number of crashes in the after period to the expected number of accidents in the before period for the treatment group, as shown in the following equation: odds ratio =
P(A|group 1)/[1 − P(A|group 1)] P(A|group 2)/[1 − P(A|group 2)]
(2)
where A is any event with probabilities P(A|group 1) and P(A|group 2). The 95% confidence intervals were obtained by taking the natural log of the odds ratio ± 1.96 times the standard error (Ott and Longnecker, 2001). 4. Results Crash characteristics are shown in Table 1. As shown, the crash rate for all age groups was lower after implementation of GDL than before. All teenage groups experienced a larger decrease in the crash rate than the 35–44-year-old age group (21.6%), except for 15-year-old who experienced a smaller reduction (15.3%). All decreases were statistically significant at the 95% level of significance using a test of proportionality in the following equation (Ott and Longnecker, 2001): z=
ˆ1 − ˆ2 ˆ 1 (1 − ˆ 1 )/n1 ) + ( ˆ 2 (1 − ˆ 2 )/n2 ) (
(3)
Table 1 Characteristics of crashes Age group
14-year olds 15-year olds 16-year olds 17-year olds 35–44-year olds
Crashes
Crash rate (crashes/licensed driver)
Before
After
Before
After
1,219 3,425 27,477 24,336 91,791
719 2,816 17,740 11,998 66,967
0.017 0.027 0.182 0.154 0.055
0.009 0.023 0.127 0.119 0.043
Decrease (%)
Odds ratio (95% confidence interval)
42.9 15.3 30.2 25.3 21.6
0.81 (0.74, 0.89) 1.13 (1.07, 1.19) 0.89 (0.87, 0.90) 0.68 (0.66, 0.69)
1404
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
Table 2 Crashes by license type for 15-year-old drivers 1996
1997
1998
15-year-old drivers involved in crashes by license type Minor school license 442 (64.2%) Instruction permit 247 (35.8%)
421 (67.9%) 199 (32.1%)
Number of 15-year-old licensed drivers Total licensed drivers 31,990 Minor school license Data not available
31,990 Data not available
Crash rate per 1000 licensed drivers Minor school license N/A Instruction permit N/A
N/A N/A
2001
2002
2003
2004
408 (70.7%) 194 (29.3%)
370 (70.6%) 154 (29.4%)
430 (70.7%) 178 (29.3%)
428 (71.5%) 171 (25.7%)
463 (73.8%) 164 (26.2%)
31,287 6,690 (21.4%)
31,038 7,106 (22.9%)
30,287 7,342 (24.2%)
30,819 7,925 (25.7%)
31,617 8,456 (26.7%)
61.0 6.9
52.1 6.4
58.6 7.8
54.0 7.5
54.8 7.1
8.1
7.5
7.2
7.7
Likelihood of school license holder to be involved in a crash compared to instruction permit holder N/A N/A N/A 8.9 Crash data were not available by license type for 1995
ˆ is the sample proportion (number of where z is the z-statistic, successes divided by sample size (n)), and n is the sample size. The odds ratio for each age group is also shown in Table 1 and Eq. (3). An odds ratio less than one suggest that there are proportionally fewer crashes in the treatment group than the control group. Evaluation of the 14-, 16-, and 17-year-old driver age groups resulted in odds ratios less than one, indicating that GDL was effective in reducing crashes for these age groups. The odds ratio for the 15year-old age group was 1.13, suggesting that when accounting for factors not related to the GDL program, crashes increased. 5. Effect of school license GDL appears to have been successful for 14-, 16-, and 17-yearold drivers who experienced a reduction in crash rate per licensed driver even after adjusting for the decrease in crash rate for middleage drivers, which was used to reflect an overall downward trend in crash rate. The crash rate for 15-year-old drivers decreased at a much smaller rate than the decrease experienced by middle-age drivers from the before to after period suggesting that the crash rate for 15year-old drivers would actually have increased from the before to after period had other downward trends not influencing the number of crashes. One factor that may have impacted the effectiveness of GDL in reducing the crash rate for younger drivers is the influence of school licenses. In Iowa, teenage drivers age 14.5–16 who have instruction permits may obtain a minor school license. This license allows them to drive unaccompanied to and from school or school-sanctioned extracurricular activities. Crashes by license type are shown in Table 2 for 15-year-old drivers by year from 1996 to 1998 for the before period and 2001–2004 for the after analysis period. Information about the type of license held was not available in the 1995 crash data set. The number of 15-year-old licensed drivers and the number of those who hold a minor school license are provided for 1998 and 2001–2004. Only estimated values existed for 1997 and 1996 and no information
was available about the number of minor school licenses held for 1995. As a result, it was not possible to compare the change in crash rate for minor school license holders from the before to after period. As shown, 15-year-old instruction permit holders accounted for 21.4–26.7% of the license holders and made up 70. 5–73.8% of 15-year-old drivers involved in crashes where the license type was known (1998 and 2001–2004 values). As a result, 15-year-old drivers with minor school licenses were involved in 7.2–8.9 times more crashes than 15-year-old operating under a regular instruction permit. Crash rate by license type is also shown in Table 2. There appears to be decreases in the various metrics shown in Table 2 when comparing 1998 values to 2001–2004; although using only one data point for the before period is not conclusive. However, crashes in Iowa decreased overall by about 20% from the before to after period so decreases shown here are consistent with what was occurring in other crash statistics. Information about sanctions and moving violation convictions was also obtained for 14- and 15-year-old drivers. This information was only available from the Iowa Department of Motor Vehicles for the 6-month period from September 2004 to April 2005. As shown in Table 3, 14- and 15-year-old school license holders are 7.7 times more likely to have one or more sanctions than their peers with a regular instruction permit. They are also 4.8 times more likely to receive one or more moving convictions. It should also be noted that teenage drivers with minor school licenses have their minor school licenses suspended after one conviction and revoked after two. As a result, there are few school license holders left after one conviction. School license holders are only likely to have two or more convictions if they committed the convictions in fairly quick succession before their minor school license was suspended or revoked. Regular instruction permit holders who receive convictions attend a remedial driver interview which provides consequences but not necessarily suspension or revocation of their license. The fact that it is difficult for school license holders to get two or more violations and yet their numbers are still similar to those with instruction permits suggests that a school license holder may be racking up violations in short amounts of time.
Table 3 Sanctions and moving violations for 14- and 15-year-old drivers by license type (September 2004–April 2005) License Type
None
1
Number of driver sanctions Instruction permit School license
2
3+
Total
48,799 (99.1%) 8,068 (92.8%)
341 (0.7%) 541 (6.2%)
65 (0.1%) 59 (0.7%)
53 (0.1%) 25 (0.3%)
49,258 8,693
Number of moving violation convictions Instruction permit 48,453 (98.4%) School license 8,009 (92.1%)
524 (1.1%) 545 (6.3%)
189 (0.4%) 121 (1.4%)
92 (0.2%) 18 (0.2%)
49,258 8,693
S.L. Hallmark et al. / Accident Analysis and Prevention 40 (2008) 1401–1405
The authors realize that the above calculations do not account for exposure. Teenagers with a minor school license may be more likely to drive more frequently and for longer distances than their counterparts with a regular instruction permit. However, there was no reasonable method for obtaining exposure to account for this difference. The group was too small to adequately use the induced exposure method. It is expected that exposure accounts for some of the increase in crashes and violations for minor school license holders. Regardless, however, the point of a GDL program is to allow for teenage drivers to learn the process of driving in stages under the supervision of a responsible adult. Since many of the trips that a teenager takes are likely to be school and school-activity related, the minor school license allows inexperienced teenagers to negate the purpose of a GDL by allowing them to drive unsupervised. There is also currently no way for an officer who pulls over a teenager who is driving alone under a school license to verify whether the teenager is enroute to school or a school-sanctioned activity. As a result, this group of teenage drivers may be spending a significant time driving alone even without attending school activities. As shown in Table 2, 15-year-old drivers with minor school permits are consistently involved in significantly more crashes than their peers with instruction permits. 6. Summary and conclusions The effectiveness of the Iowa graduated drivers licensing program was evaluated for a 4-year period before and after implementation in 1999. Since some changes had occurred in the crash reporting form and the types of crashes which are required to be reported, changes in crash rates for younger drivers were compared to the reduction in crash rate for 35–44-year-old drivers who are assumed to represent a stable group and were used to reflect general downward trends. After implementation of GDL, the crash rate for all age groups in the after period was lower than in the before period. The teenage groups (14-, 16-, and 17-year-old) experienced a greater decrease in crash rate than the 35–44-year-old age group while the 15-year-old experienced a smaller decrease. This suggests that the crash rate for 15-year-old drivers actually increases when downward trends are adjusted for. The results for the 15-year-old age group were unexpected. The rate of compliance to GDL restrictions for any age group is unknown
1405
so the effect of compliance could not be evaluated. As indicated, those with minor school licenses were far more likely to be involved in a crash as well as have more moving violations and convictions. The prevalence of the minor school license may have been the major factor that resulted in failure of GDL to reduce crashes for 15-yearold. Results suggest that room for improvement exists in the Iowa GDL program. The most obvious would be to reconsider granting minor school licenses. Acknowledgement The research team wishes to thank the Iowa Department of Transportation for funding this project. References Baker, S.P., Chen, L.-H., Li, G., 2007. Nationwide Review of Graduated Driver Licensing. AAA Foundation for Traffic Safety, Washington, DC. Harwood, D.M., Bauer, K.M., Potts, I.B., Torbic, D.J., Richard, K.R., Kohlman Rabbani, E.R., Hauer, E., Elefteriaduou, L., 2002. Safety effectiveness of intersection leftand right-turn lanes. Federal Highway Administration, Office of Safety Research Development. FHWA-RD-02-089. Hauer, E., 1997. Observational Before–After Studies in Road Safety. Pergamon, New York, 1997. Institute for Highway Safety. Licensing System for Young Drivers. http://www. iihs.org/laws/graduatedLicenseIntro.aspx. Accessed November 2007. Iowa Office of Driver Services. Graduated Driver Licenses (GDL). Information on Iowa’s System for Drivers under the Age of 18. http://www.iamvd.com/ods/ gdl.htm. Accessed September 2007. Mayhew, D.R., Simpson, H.M., Pak, A., 2003. Changes in collision rates among novice drivers during the first months of driving. Accident Anal. Prev. 5, 683–691. National Highway Traffic Safety Association. Graduated Driver Licensing System. http://www.nhtsa.dot.gov/people/outreach/safesobr/19qp/factsheets/graduated.html. Accessed January 7, 2004. National Highway Traffic Safety Association, 2005. Traffic Safety Facts: 2005 Data, Young Drivers. Online html Document: http://www-nrd.nhtsa.dot.gov/ Pubs/810630.PDF. Accessed November 2007. Ott, R.L., Longnecker, M., 2001. An Introduction to Statistical Methods and Data Analysis. Duxbury, Thomson Learning, Pacific Grove, CA. Shope, J.T., 2007. Graduated Driver Licensing: Review of Evaluation Results Since 2002. J. Safety Res. 38, 165–175. Shope, J.L., Molnar, L.J., 2003. Graduated driver licensing in the United States: evaluation results from the early programs. J. Safety Res. 34, 63–69. Shope, J.L., Molnar, L.J., 2004. Michigan’s graduated driver licensing program: evaluation of the first four years. J. Safety Res. 35, 337–344. Ulmer, R.G., Preusser, D.F., Williams, A.F., Ferguson, S.A., Farmer, C.M., 2000. Effect of Florida’s graduated driver licensing program on the crash rate of teenage drivers. Accident Anal. Prev. 32, 527–532.