- Email: [email protected]
The effect of environmental and design parameters on subjective road safety — a case study in Poland
Safety
Science 19 (1995) 227-234
The effect of environmental and design parameters on subjective road safety - a case study in Poland L. Zakowska Cracow University of Technology, Warszawska str. 24, 31-155 Cracow, Poland
Abstract In order to get a better insight into the safety effects of alternative road designs, the subjective safety experiment has been conducted. The experiment has been designed to test the effect of geometric design characteristics and road environment on the subjective safety of rural road curves. The method used here allows to investigate the effect of a real driving environment on safety in a complex way. The results have shown the importance of the composed effect of curve geometry and curve environment on the subjective safety of rural curves.
1. Introduction Road safety, especially in rural areas, is critically influenced by the way roads are built. Although relationships between safety and road feature have been studied for nearly 50 years, clearly much remains unknown about safety and geometric design relationships. In most of safety related studies accident rates associated with different roadway designs have been estimated by using actual accident records and travel data. This approach, based on accident research, is difficult: - accidents are relatively infrequent so that deep statistical studies require consistent data collected over long periods of time for many kilometers of road, _ many factors related to the road environment, the driver and the vehicle interactively contribute to the occurrence of severity of accidents, but are seldom included in the accident data base (even with reasonably complete data bases researchers are often unable to sort out effects attributable to the specific roadway features), - estimates of accident rates developed using data from one area might not be appropriate elsewhere, because of differences in reporting practices for non fatal accidents, - some factors that underlie relationships between safety and road design change over time so that relationships developed at one time may no longer be representative in later years. 0925-7535/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10925-7535(94)00023-9
228
L. Zukowska/Safety
Science 19 (1995) 227-234
In order to get better insight into the safety effects of alternative road designs, the subjective safety experiment has been designed and conducted.In this experimental design another approach for safety measure was used. The subjective road safety assessed by the drivers reflects drivers subjectively perceived level of safety of the road situation. The experiment has primarily been designed to test the effect of geometric road design elements and road environment on the subjective safety of rural road curves. Accidents are more likely to occur on horizontal curves than on straight segments of roadway, because of increased demands placed on the driver and the vehicle. Over 50% of the total number of accidents on rural two-lane roads involve single vehicle accidents on curves. The safety effects of an individual curve are influenced not only by the curve’s geometric characteristics, but also by other factors related to the road environment, driver’s behavior and perception, and many other factors. The estimation of the combined effects on safety of roadview related factors was the main objective of this study. The results of the experiment provide more insight into the nature and extent of the risk produced by the road curve’s perceptible manipulated characteristics, which almost always occur in combinations.
2. Experimental design A multifactorial experiment has been designed to test the effect of road curve characteristics, road environment and driver’s experience on the subjective safety of rural two-lane roads. 2.1. Method Forty licensed drivers of different driving experience were tested in laboratory conditions. Subjects were asked to give a rating of the presented road situation (the approaching zone of the curve and the curve itself with different geometric and environmental characteristics) that reflects the subjectively perceived level of safety. The subjective rating scale was used as safety measure. An important assumption underlying the use of this measure is that subjects are able to verbalize the amount of perceived safety in an adequate way. Ganton and Wilde ( 1971)) performing an experiment in order to test the usefulness of a verbal rating scale of estimated danger, assumed that drivers are sufficiently aware of the change of estimated danger to be able to report it verbally. Wilde (1982) concluded that subjects are able, when asked, to express their perceived level of risk. Moran ( 1983) investigated the relationship between objective safety and subjectively perceived safety As a measure of objective safety the numbers of accidents per lo6 vehicle/km travelled was evaluated for each road section. Subjective safety was evaluated by asking the subjects to estimate the amount of perceived safety in the tested situation. It was found that the objective accident risk correlated positively with subjective risk rating per unit distance. Grant ( 1985) stated that verbal rating measure constitutes a cognitive representation of the concept of perceived safety.
L. Zukowska/Safety
the
very poor
1
Science 19 (1995) 227-234
road
safety
+ Fig.
229
+
very good
1.Rating scale used in the experiment.
2.2. Rating scale Different rating scales with respect to the number of scale points and the definition of scale values were used by different experimenters: Canton and Wilde ( 1971), Browning at al. ( 1977), Moran (1983), Grant and Wilde (1985),Huddart (1978),Zakowska (1989). Taking together the psychometric advantages and psychological disadvantages of many scale points, a rating scale of 7 scale points was selected as the most appropriate for this experiment. A second property of rating scales concerns the distinction between graphic and numerical scales. For several reasons, the graphic scale is preferable to the use of numbers without the graphic scale (Nunnally, 1978). With the graphic scale employed in this experiment (Fig. 1) the subject has to mark the appropriate box, corresponding to the numerical scale that could be defined as follows: 1: unacceptable risk perceived, 2: very much risk perceived, 3: much risk perceived, 4: intermediate risk perceived, 5: little risk perceived, 6: very little risk perceived, 7: no risk perceived. Here only extreme scale points are defined. The interpretation of the undefined scale points was left to the driver, assuming that the distance between the scale points had to be considered as equal. 2.3. Stimulus material Two-lane rural highways in southern Poland were chosen as an experimental polygon. The approaching zones to the selected horizontal curves (about 300 meters of long straight segments before a curve) together with the curves were filmed on video as moving perspective road view observed from the driver’s eye position. The perspective viewpoint was the front seat viewing point of a driver in a vehicle positioned in the right-hand lane, with 1.20 m eye height, as specified by the National Standards in Poland (the similar specifications to those recommended in RAAL and AASHTO). The vehicle speed was the same as the designed speed for each site. To eliminate the influence of non manipulated external factors, all sites were filmed in summer driving conditions, good light and weather, and none or limited traffic on the road. Curve geometric features (curve radius and deflection angle), characteristic of road environment and curve direction were manipulated in the experiment. Ten levels of curve
230
L. Zukowska /Safety Science 19 (1995) 227-234
geometry (radii of 150 m, 300 m, 500 m and 700 m crossed with angles of 9, 18, 36 and 72”)) 2 curve directions, 2 categories of road environment (opened and walled roads) and 2 levels of viewing distance to curve (6 s and 3 s drive to curve) produced 80 stimuli that were presented in the preorganized order, in four sessions of 20 road views each. 2.4. Subjects 40 Licensed drivers took part in this experiment. They were selected and tested in four groups of 10 drivers of different driving experience. The groups were classified as follows: 1 - experienced road designers and drivers (over 100,000 km driven, and at least 10 years of experience in road designing), 2 - very experienced drivers = professional drivers (min. 300,000 km driven and min. 10 years of practice), 3 -experienced drivers (20,000-100,000 km driven, min. 5 years driving), 4 - unexperienced drivers (less than 10,000 km driven, max. 2 years of holding driving license). 2.5. Procedure The group presentations were used in this experiment. Subjects were seated in a laboratory in front of a 28” color monitor screen, having a good view on the screen, and they were discouraged from communicating during the experiment. A pre-recorded tape of experimental instructions and the practice film were presented as a training at the start of the session. Then the sequences of 20 film-clips were presented, with 30 s of blank film between each clip, when subjects made their assessments on the response booklet provided beforehand. The whole experiment consisted of 16 sessions, each about 45 minutes long.
3. Results Safety estimates were scored (from 1 - for very unsafe, to 7 - for very safe), recorded and analyzed using analysis of variance and the omega-squared statistics. The latter measure allows estimation of the strength of statistical relationship in terms of the percentage of variance, accounted for by a particular manipulation (Hays,1984). Statistical effect can be graded in terms of importance on the basis of the omega-square value (here,the value is shown as a percentage of an explained variance). Using this approach, the stronger effects can be selected for more detailed attention. In these studies, only significant effects explaining more than 0.5 percent of the variance were considered. All significant effects and interactions are listed in Table 1. Applying this criterion, the only main effect of curve geometry and 9 interactions need to be examined. Curve geometry was the only significant main effect (22.50% explained variance, p < 0.0001). The highest range interactions also included manipulations of curve geometry. Additional studies have been performed to analyze the effect of curve geometry more
L. Zukowska /Safety Science 19 (1995) 227-234
Table
231
I
ANOVA
summary table of significant effects
Source of variation
ss
Geometry Geom. &dir.
& envir.
F-ratio
MS
O.l885D+4
O.l885D+3
O.l49D+3
Explained variance
F prob.
(So)
(%)
22.50
0.0000 0.0000 0.0000 0.0000 0.0000
0.4094D
+ 3
0.4904D + 2
0.456D + 2
4.80
Geom. & direction
0.4084D
+ 3
0.4084D + 2
0.347D + 2
4.77
Envir. & geom.
0.28 14D + 3
0.2814D+2
0.249D+2
3.25
Distance & geom.
0.9954D
+ 2
0.9954D +
I
0.742D +
I
Geom. & dir. & dist.
0.6765D
+ 2
0.6765D +
0.6167D+2
0.6167D +
1 I
0.600D +
Geom. & envir. & dist.
0.544D +
Geom. & experience
0.8949D
+ 2
0.2983D + 1
0.236D +
Geom. & exper. & envir.
0.7628D
+ 2
0.2543D +
I
0.225D +
I I 1 I
Envir. & direction
0.4609D
+ 2
0.46091)+2
subjective safety
0.195D+2
1.04 0.68
0.0002
0.61
0.0006
0.62
0.0233
0.5
1
0.0447
0.53
0.0230
6 5 4 3 0
-
right
l
-
left
obtl
curvem curve*
wa; led
environment
Fig. 2. The effect of curve direction and the character of road environment on the subjective safety of curves
detailed. This involved manipulation of curve radius alone, with the constant level of angle and curve angle manipulation, when the radius level is not changed. Among other interactions, very interesting was an interaction of curve direction and the character of road environment (Fig. 2). Subjects found open curves more safe than walled ones, where the inner edge of the curve is not visible. This distinction was stronger for righthand than for left-hand curves, regardless the variable curve geometry.
4. Curve geometry manipulations When manipulating curve deflection angle for small curves’ radii (150 m) angle was the only significant source of subjective safety variation (17% explained variance,p < 0.0001). An interesting interaction of curve angle and driver’s experience was also significant (for open curves: 5.7% expl.var., p < 0.001, and for walled curves: 2.1% expl.var., p < 0.05).
232
L. ZukowskalSafety
Science 19 (1995) 227-234
R= 15OP
mean subjective safety
o
-
b
-
x +
-
left,open rlght.walled left,wallsd
angle
curve
Fig. 3. The effect of curve angle, curve direction and curve environment
rlqht.open
on the subjective safety of small radii
curves.
Curve angle proved to be an important factor decisive for the level of subjectively perceived risk of small radii curves. This effect is strongly influenced by the character of the environment, as well as by the curve direction (Fig. 3). Angle was also significant for 300 m radii curves (40% expkvar., p
subjective safety Em
6 ;
1
~1
,“::::td
go
I I
1s” I
Fig. 4. The effect of curve angle and the character curves.
39
I
of curve environment
72 I
curve
angle
on the subjective safety of 300 m radii
L. Zrkowska/Safety Science 19 (199s) 227-234
mean
7
subjective safety
+I
l
6
X + 0
5
cathegor1efI:
X
*
l
2 1
AI Fig. 5. The effect of driving
drlver’a
0
4 3
233
1
experience
curve
0
-
exp.desIgnem
+
-
v.experlenced
x
1
0
-
exparlenced
-
mexpsr
1enced
l&e
angle
and curve angle on the subjective safety of 500 m radii curves
7 6
mean subjective safety
5
I I
I
open
walled
Fig. 6. The effect of road environment
mean subjective safety
on the perceived safety of curves
7
6 5 4
-
o-.--O
3 2 1
I 150I
Fig. 7. The effect of radius on subjective
I I 300
I 500
safety (curves of all environment
I 700
radius
and angles manipulated
[ml through the
experiment).
-radius (13.5%expl.var.,p<0.0001), and -environment (2.8% expl.var.,p
234
L. Zukowska/Safety Science I9 (1995) 227-234
as presented in Fig. 7. Also radius affected the subjective neglected, if compared with the angle effect.
safety strong enough not to be
5. Conclusions This experiment has shown that drivers are able, based on motion road views, to discriminate different levels of perceived safety of curves. The results revealed the complexity of the effects on safety of rural curves. A general conclusion of this experiment is that road environment constitutes an important factor of curve safety estimation. The main effect concerning curve angle and radius effect on subjective safety supports the findings from previous studies performed in a driving simulator (Tenkink and Horst, 1991). The advantage of the method used in this experiment (employing filmed road views) in relation to a driving simulator is, that it allows to study the effect of real world driving environment on safety in a more complex way. Difficulties in selecting and controlling the stimuli set, however, may result sometimes in lowering the accuracy of results.
References Fildes, B.N., Leening, A.C. and Corrigan, J., 1989. Speed perception 2; Driver’s judgments of safety and speed on rural curved roads and for different following distances. Federal Office of Road Safety, RACV, CR 60. Hays, W.L., 1984. Statistics. The University of Texas at Austin, Holt, Rinehart and Winston. Huddart, L., 1978. An evaluation of the visual impact of rural roads and traffic. TRRL Suppl. Report No. 355, Crowthorne, Berkshire. Riemersma, J.B.J., 1984. Driving hehaviour in road curves. TN0 Institute for Perception, Rep. No. IZF C-12. Tenking, E. and Horst, A.R.A. van der, 199I, Effects of road width and curve characteristics on driving speed. TN0 Institute for Perception, Rep. No. IZF C-26. TRB Special Report, 1987. Designing safer roads, National Research Council, Washington, DC.
Science 19 (1995) 227-234
The effect of environmental and design parameters on subjective road safety - a case study in Poland L. Zakowska Cracow University of Technology, Warszawska str. 24, 31-155 Cracow, Poland
Abstract In order to get a better insight into the safety effects of alternative road designs, the subjective safety experiment has been conducted. The experiment has been designed to test the effect of geometric design characteristics and road environment on the subjective safety of rural road curves. The method used here allows to investigate the effect of a real driving environment on safety in a complex way. The results have shown the importance of the composed effect of curve geometry and curve environment on the subjective safety of rural curves.
1. Introduction Road safety, especially in rural areas, is critically influenced by the way roads are built. Although relationships between safety and road feature have been studied for nearly 50 years, clearly much remains unknown about safety and geometric design relationships. In most of safety related studies accident rates associated with different roadway designs have been estimated by using actual accident records and travel data. This approach, based on accident research, is difficult: - accidents are relatively infrequent so that deep statistical studies require consistent data collected over long periods of time for many kilometers of road, _ many factors related to the road environment, the driver and the vehicle interactively contribute to the occurrence of severity of accidents, but are seldom included in the accident data base (even with reasonably complete data bases researchers are often unable to sort out effects attributable to the specific roadway features), - estimates of accident rates developed using data from one area might not be appropriate elsewhere, because of differences in reporting practices for non fatal accidents, - some factors that underlie relationships between safety and road design change over time so that relationships developed at one time may no longer be representative in later years. 0925-7535/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10925-7535(94)00023-9
228
L. Zukowska/Safety
Science 19 (1995) 227-234
In order to get better insight into the safety effects of alternative road designs, the subjective safety experiment has been designed and conducted.In this experimental design another approach for safety measure was used. The subjective road safety assessed by the drivers reflects drivers subjectively perceived level of safety of the road situation. The experiment has primarily been designed to test the effect of geometric road design elements and road environment on the subjective safety of rural road curves. Accidents are more likely to occur on horizontal curves than on straight segments of roadway, because of increased demands placed on the driver and the vehicle. Over 50% of the total number of accidents on rural two-lane roads involve single vehicle accidents on curves. The safety effects of an individual curve are influenced not only by the curve’s geometric characteristics, but also by other factors related to the road environment, driver’s behavior and perception, and many other factors. The estimation of the combined effects on safety of roadview related factors was the main objective of this study. The results of the experiment provide more insight into the nature and extent of the risk produced by the road curve’s perceptible manipulated characteristics, which almost always occur in combinations.
2. Experimental design A multifactorial experiment has been designed to test the effect of road curve characteristics, road environment and driver’s experience on the subjective safety of rural two-lane roads. 2.1. Method Forty licensed drivers of different driving experience were tested in laboratory conditions. Subjects were asked to give a rating of the presented road situation (the approaching zone of the curve and the curve itself with different geometric and environmental characteristics) that reflects the subjectively perceived level of safety. The subjective rating scale was used as safety measure. An important assumption underlying the use of this measure is that subjects are able to verbalize the amount of perceived safety in an adequate way. Ganton and Wilde ( 1971)) performing an experiment in order to test the usefulness of a verbal rating scale of estimated danger, assumed that drivers are sufficiently aware of the change of estimated danger to be able to report it verbally. Wilde (1982) concluded that subjects are able, when asked, to express their perceived level of risk. Moran ( 1983) investigated the relationship between objective safety and subjectively perceived safety As a measure of objective safety the numbers of accidents per lo6 vehicle/km travelled was evaluated for each road section. Subjective safety was evaluated by asking the subjects to estimate the amount of perceived safety in the tested situation. It was found that the objective accident risk correlated positively with subjective risk rating per unit distance. Grant ( 1985) stated that verbal rating measure constitutes a cognitive representation of the concept of perceived safety.
L. Zukowska/Safety
the
very poor
1
Science 19 (1995) 227-234
road
safety
+ Fig.
229
+
very good
1.Rating scale used in the experiment.
2.2. Rating scale Different rating scales with respect to the number of scale points and the definition of scale values were used by different experimenters: Canton and Wilde ( 1971), Browning at al. ( 1977), Moran (1983), Grant and Wilde (1985),Huddart (1978),Zakowska (1989). Taking together the psychometric advantages and psychological disadvantages of many scale points, a rating scale of 7 scale points was selected as the most appropriate for this experiment. A second property of rating scales concerns the distinction between graphic and numerical scales. For several reasons, the graphic scale is preferable to the use of numbers without the graphic scale (Nunnally, 1978). With the graphic scale employed in this experiment (Fig. 1) the subject has to mark the appropriate box, corresponding to the numerical scale that could be defined as follows: 1: unacceptable risk perceived, 2: very much risk perceived, 3: much risk perceived, 4: intermediate risk perceived, 5: little risk perceived, 6: very little risk perceived, 7: no risk perceived. Here only extreme scale points are defined. The interpretation of the undefined scale points was left to the driver, assuming that the distance between the scale points had to be considered as equal. 2.3. Stimulus material Two-lane rural highways in southern Poland were chosen as an experimental polygon. The approaching zones to the selected horizontal curves (about 300 meters of long straight segments before a curve) together with the curves were filmed on video as moving perspective road view observed from the driver’s eye position. The perspective viewpoint was the front seat viewing point of a driver in a vehicle positioned in the right-hand lane, with 1.20 m eye height, as specified by the National Standards in Poland (the similar specifications to those recommended in RAAL and AASHTO). The vehicle speed was the same as the designed speed for each site. To eliminate the influence of non manipulated external factors, all sites were filmed in summer driving conditions, good light and weather, and none or limited traffic on the road. Curve geometric features (curve radius and deflection angle), characteristic of road environment and curve direction were manipulated in the experiment. Ten levels of curve
230
L. Zukowska /Safety Science 19 (1995) 227-234
geometry (radii of 150 m, 300 m, 500 m and 700 m crossed with angles of 9, 18, 36 and 72”)) 2 curve directions, 2 categories of road environment (opened and walled roads) and 2 levels of viewing distance to curve (6 s and 3 s drive to curve) produced 80 stimuli that were presented in the preorganized order, in four sessions of 20 road views each. 2.4. Subjects 40 Licensed drivers took part in this experiment. They were selected and tested in four groups of 10 drivers of different driving experience. The groups were classified as follows: 1 - experienced road designers and drivers (over 100,000 km driven, and at least 10 years of experience in road designing), 2 - very experienced drivers = professional drivers (min. 300,000 km driven and min. 10 years of practice), 3 -experienced drivers (20,000-100,000 km driven, min. 5 years driving), 4 - unexperienced drivers (less than 10,000 km driven, max. 2 years of holding driving license). 2.5. Procedure The group presentations were used in this experiment. Subjects were seated in a laboratory in front of a 28” color monitor screen, having a good view on the screen, and they were discouraged from communicating during the experiment. A pre-recorded tape of experimental instructions and the practice film were presented as a training at the start of the session. Then the sequences of 20 film-clips were presented, with 30 s of blank film between each clip, when subjects made their assessments on the response booklet provided beforehand. The whole experiment consisted of 16 sessions, each about 45 minutes long.
3. Results Safety estimates were scored (from 1 - for very unsafe, to 7 - for very safe), recorded and analyzed using analysis of variance and the omega-squared statistics. The latter measure allows estimation of the strength of statistical relationship in terms of the percentage of variance, accounted for by a particular manipulation (Hays,1984). Statistical effect can be graded in terms of importance on the basis of the omega-square value (here,the value is shown as a percentage of an explained variance). Using this approach, the stronger effects can be selected for more detailed attention. In these studies, only significant effects explaining more than 0.5 percent of the variance were considered. All significant effects and interactions are listed in Table 1. Applying this criterion, the only main effect of curve geometry and 9 interactions need to be examined. Curve geometry was the only significant main effect (22.50% explained variance, p < 0.0001). The highest range interactions also included manipulations of curve geometry. Additional studies have been performed to analyze the effect of curve geometry more
L. Zukowska /Safety Science 19 (1995) 227-234
Table
231
I
ANOVA
summary table of significant effects
Source of variation
ss
Geometry Geom. &dir.
& envir.
F-ratio
MS
O.l885D+4
O.l885D+3
O.l49D+3
Explained variance
F prob.
(So)
(%)
22.50
0.0000 0.0000 0.0000 0.0000 0.0000
0.4094D
+ 3
0.4904D + 2
0.456D + 2
4.80
Geom. & direction
0.4084D
+ 3
0.4084D + 2
0.347D + 2
4.77
Envir. & geom.
0.28 14D + 3
0.2814D+2
0.249D+2
3.25
Distance & geom.
0.9954D
+ 2
0.9954D +
I
0.742D +
I
Geom. & dir. & dist.
0.6765D
+ 2
0.6765D +
0.6167D+2
0.6167D +
1 I
0.600D +
Geom. & envir. & dist.
0.544D +
Geom. & experience
0.8949D
+ 2
0.2983D + 1
0.236D +
Geom. & exper. & envir.
0.7628D
+ 2
0.2543D +
I
0.225D +
I I 1 I
Envir. & direction
0.4609D
+ 2
0.46091)+2
subjective safety
0.195D+2
1.04 0.68
0.0002
0.61
0.0006
0.62
0.0233
0.5
1
0.0447
0.53
0.0230
6 5 4 3 0
-
right
l
-
left
obtl
curvem curve*
wa; led
environment
Fig. 2. The effect of curve direction and the character of road environment on the subjective safety of curves
detailed. This involved manipulation of curve radius alone, with the constant level of angle and curve angle manipulation, when the radius level is not changed. Among other interactions, very interesting was an interaction of curve direction and the character of road environment (Fig. 2). Subjects found open curves more safe than walled ones, where the inner edge of the curve is not visible. This distinction was stronger for righthand than for left-hand curves, regardless the variable curve geometry.
4. Curve geometry manipulations When manipulating curve deflection angle for small curves’ radii (150 m) angle was the only significant source of subjective safety variation (17% explained variance,p < 0.0001). An interesting interaction of curve angle and driver’s experience was also significant (for open curves: 5.7% expl.var., p < 0.001, and for walled curves: 2.1% expl.var., p < 0.05).
232
L. ZukowskalSafety
Science 19 (1995) 227-234
R= 15OP
mean subjective safety
o
-
b
-
x +
-
left,open rlght.walled left,wallsd
angle
curve
Fig. 3. The effect of curve angle, curve direction and curve environment
rlqht.open
on the subjective safety of small radii
curves.
Curve angle proved to be an important factor decisive for the level of subjectively perceived risk of small radii curves. This effect is strongly influenced by the character of the environment, as well as by the curve direction (Fig. 3). Angle was also significant for 300 m radii curves (40% expkvar., p
subjective safety Em
6 ;
1
~1
,“::::td
go
I I
1s” I
Fig. 4. The effect of curve angle and the character curves.
39
I
of curve environment
72 I
curve
angle
on the subjective safety of 300 m radii
L. Zrkowska/Safety Science 19 (199s) 227-234
mean
7
subjective safety
+I
l
6
X + 0
5
cathegor1efI:
X
*
l
2 1
AI Fig. 5. The effect of driving
drlver’a
0
4 3
233
1
experience
curve
0
-
exp.desIgnem
+
-
v.experlenced
x
1
0
-
exparlenced
-
mexpsr
1enced
l&e
angle
and curve angle on the subjective safety of 500 m radii curves
7 6
mean subjective safety
5
I I
I
open
walled
Fig. 6. The effect of road environment
mean subjective safety
on the perceived safety of curves
7
6 5 4
-
o-.--O
3 2 1
I 150I
Fig. 7. The effect of radius on subjective
I I 300
I 500
safety (curves of all environment
I 700
radius
and angles manipulated
[ml through the
experiment).
-radius (13.5%expl.var.,p<0.0001), and -environment (2.8% expl.var.,p
234
L. Zukowska/Safety Science I9 (1995) 227-234
as presented in Fig. 7. Also radius affected the subjective neglected, if compared with the angle effect.
safety strong enough not to be
5. Conclusions This experiment has shown that drivers are able, based on motion road views, to discriminate different levels of perceived safety of curves. The results revealed the complexity of the effects on safety of rural curves. A general conclusion of this experiment is that road environment constitutes an important factor of curve safety estimation. The main effect concerning curve angle and radius effect on subjective safety supports the findings from previous studies performed in a driving simulator (Tenkink and Horst, 1991). The advantage of the method used in this experiment (employing filmed road views) in relation to a driving simulator is, that it allows to study the effect of real world driving environment on safety in a more complex way. Difficulties in selecting and controlling the stimuli set, however, may result sometimes in lowering the accuracy of results.
References Fildes, B.N., Leening, A.C. and Corrigan, J., 1989. Speed perception 2; Driver’s judgments of safety and speed on rural curved roads and for different following distances. Federal Office of Road Safety, RACV, CR 60. Hays, W.L., 1984. Statistics. The University of Texas at Austin, Holt, Rinehart and Winston. Huddart, L., 1978. An evaluation of the visual impact of rural roads and traffic. TRRL Suppl. Report No. 355, Crowthorne, Berkshire. Riemersma, J.B.J., 1984. Driving hehaviour in road curves. TN0 Institute for Perception, Rep. No. IZF C-12. Tenking, E. and Horst, A.R.A. van der, 199I, Effects of road width and curve characteristics on driving speed. TN0 Institute for Perception, Rep. No. IZF C-26. TRB Special Report, 1987. Designing safer roads, National Research Council, Washington, DC.