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Further study of the composition of the Dutch car fleet
the Science o f t h e Total Environment ELSEVIER
The Science of the Total Environment 145 (1994) 235-242
Further study of the composition of the Dutch car fleet ..a
A . H . Versluls
, M.E. van den Bol-de J o n g b
aRoad and Hydraulic Engineering Division. Directorate General for Public Works and Water Management, P.O. Box 5044, 2600 GA Delft, The Netherlands hCentre for Applied Statistics, TNO Institute of Applied Physics, P.O. Box 155, 2600 AD Delft, The Netherlands
(Received 4 February 1993; accepted 2 March 1993)
Abstract This article discusses the results of an investigation into the composition of the Dutch car fleet with regard to salient vehicle characteristics such as fuel type, unladen weight and age. Data for this study were collected by recording the registration numbers of cars passing six motorway locations at three distinct times of the day. Analyses of the results have revealed significant differences in the composition of these traffic flows with respect to both place and time. It is therefore recommended that account be taken of such differences when interpreting the results of noise emission measurements and measurements of air-borne pollutants and soil contaminants emitted by passing vehicles. It has been shown, for instance, that variations in the composition of traffic flows with respect to the unladen weight of the constituent vehicles could give rise to differences in noise emissions of up to 0.8 dB(A) for the average individual car between locations and times of the day. However, it should be noted that the unladen weight of a vehicle is only one of a number of characteristics that can affect noise levels. Speed, fuel type, engine size, tyre tread and the age of vehicles are all known to influence the level of noise generated when a car travels on the road. It therefore seems reasonable to assume that any differences in the composition of traffic flows with respect to these parameters will produce even larger variations in in-situ noise emission levels than are quoted in this paper. Key words." Noise emissions; Fuel type; Passenger car fleet; Unladen weight; Vehicle age
1. Introduction The D i r e c t o r a t e G e n e r a l for Public W o r k s and W a t e r M a n a g e m e n t in the N e t h e r l a n d s regularly m o n i t o r s traffic flow on D u t c h r o a d s in o r d e r to quantify traffic noise and to determine the concentration o f air b o r n e pollutants and soil contaminants emitted by passing vehicles. It is k n o w n * Corresponding author.
that emissions o f this type can v a r y from vehicle to vehicle, which implies that noise a n d p o l l u t a n t emissions are not only d e p e n d e n t on traffic density but also on the c o m p o s i t i o n o f the traffic flow. Until now little account has been taken o f possible differences in the m a k e - u p o f traffic flows. A p a r t from a limited n u m b e r o f studies, in which consideration was given to the volume o f heavy g o o d s vehicles on the road, it has generally been assumed that a given sample could be regarded as being
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(93)03687-W
236
A.H. Versluis. M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
representative of the average car fleet in the country, provided that the traffic density was of a sufficient level at the time the measurements were made. In practice, this has meant that differences in the composition of traffic flows at various monitoring stations and times of the day have been considered to be negligible provided that the samples taken were sufficiently large. It has been recognised that considerable differences exist between the composition of national car fleets. For instance, the composition of local traffic flows in Poland and the USA is likely to differ. If such differences are apparent on an international level, it therefore seems reasonable to question assumptions about the homogeneity of traffic flows within a single country. Key vehicle characteristics that are important in this context are: fuel type, engine size, vehicle age, speed, unladen weight, tyre width and tyre tread. It was therefore decided to investigate the correctness of the assumption that the composition of traffic flows over a certain size are broadly similar throughout the Netherlands, by recording the unladen weight, fuel type and age of the vehicles passing selected monitoring stations [1]. Where possible, comparisons were made with data obtained from the national vehicles register kept by the Government Road Transport Department. For the used fleet, use was also made of information compiled by the Central Bureau of Statistics concerning the average number of kilometres travelled on Dutch roads each year by particular types of car.
2. Monitoring procedure At the end of August 1991, the Traffic Engineering Department of the Directorate General for Public Works and Water Management recorded the registration numbers of cars passing six monitoring stations along various motorways in the Netherlands. The precise location of the monitoring stations at which the visual registrations took place is given in Table 1. In choosing the six sites, a deliberate effort was made to select two examples of each of the three main types of motorway encountered in the Netherlands. Consideration was also given to maintaining an acceptable distance between the various monitoring stations and ensuring that a sufficient geographic spread was achieved. The number plates of passing cars were noted at each of the six locations 3 times/day. This involved recording vehicle registration numbers for 2 h during the morning and evening rush hours, and for a 4-h period during the day. Each registration period was divided into a series of 15-min intervals. The first 70 cars were sampled during each of the eight intervals in the two rush hour monitoring sessions, and the first 35 cars during the 16 intervals in the daytime monitoring sessions. This allowed at least 500 registration numbers to be recorded during each monitoring session. It was decided to collect such a large number of registration numbers in order to ensure that a sufficiently representative sample was obtained.
Table 1 Monitoring stations at which the registration numbers of passing vehicleswere recorded Location number
Motorway
Town
Type of roada
7 13 17 21 52 56
A59 A59 A58 A50 A 13 A29
Made Vlijmen Moergestel Ravenstein Delft-Zuid Heinenoord
Link to main road network Link to main road network Hinterland connection Main transport route Hinterland connection Main transport route
"Classification in accordancewith Ref. [2].
A.H. Versluis, M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
237
Ownership and Use of Passenger Cars Total
Dutch
car
fleet in
August
1991
80
70
60
car
ownership
use
[kin/year]
[vhc]
!
~-~ ~--~ 5 0 ~D
~
4O Total car fleet 5.244 million cars Total annual use 84.16 million km
IlJ
2O
10 Diesel Tuel type
Petrol
LPG
Fig. 1. Compostition of the Dutch car fleet based on vehicle registration data for August 1991 and on statistics regarding the average use of vehicles on Dutch roads.
Number
of csrs reco:ded
80
by the Traffic
August
Engmeer:ng
De~ar-Lmen[
1991
90
t
60
513
40
& 30
20
10
Petrol 7
13
19
21
O l e 519 I 52
56
?
13
17
21
52
L PC~ 56
?
13
in
21
~uel 52
56
type
t_o~-a b a n
Fig. 2. Comparison of the composition of traffic flows at the six monitoring stations with respect to fuel type and in relation to the average use of cars on Dutch roads.
238
A.H. Versluis, M.E. van den Bol-de Jong et al./Sci. Total Environ. 145 (1994) 235-242
3. Results 3.1. Traffic f l o w composition with respect to f u e l type
It is important to note that when discussing traffic flow measurements, a distinction should be made between the composition of the national car fleet as based on vehicle registrations and the composition obtained from data on vehicle use. For the purpose of practical environmental research, measurements are normally made of actual traffic flows, which should reflect statistics on vehicle usage. Fig. 1. illustrates the considerable differences that exist between the composition of the national car fleet in terms of ownership and the use of particular types of vehicle in the Netherlands. The data shown in Fig. 1 contrast the average distance travelled in petrol, diesel and liquid peteroleum gas (LPG) powered cars with the number of such vehicles registered in the Netherlands in August 1991. For comparison purposes, both distributions are expressed on a percentage basis. Fig. 1 shows that the breakdown of the national car fleet in terms of ownership is different from the
composition obtained on the basis of vehicle use and that diesel and L P G powered cars are used more frequently than petrol driven cars. In the present study, the registration numbers of - 500 cars were recorded at the six selected monitoring stations at three different times of the day. The composition of the vehicle registration numbers was analysed statistically using log-linear models [6-8]. It was shown that while the composition of the traffic flow with regard to fuel type did not differ significantly at various times of the day, significant differences were found with regard to the make-up of the traffic flow passing the six selected monitoring stations. The breakdown of the traffic flow with regard to fuel type for the six locations is shown in Fig. 2, together with the composition calculated on the basis of average vehicle usage figures compiled with data from the Government Road Transport Department and the Central Bureau of Statistics (horizontal line) [3,4]. The data on the type of fuel used in the cars monitored at the different locations have been expressed in terms of a mean value and associated 95% confidence levels. In each case,
Ownership and Use of Passenger Cars Total Dutch car
f l e e t in A u g u s t
1991
25 car ownership [vhc] I '+ use [krn/year ]
20
C
~1o Total car fleet ,5.244 million cars Total annual use 84.16 million km
13..
1 tO0 1300 1000 1200 >1350 Unladen Weight [kg] 9OO
<7 800
Fig. 3. Composition of the national Dutch car fleet categorised according to unladen weight which is based on ownership data for August 1991 and average usage in 1991.
239
A.H. Versluis, M.E. van den Bol-de Jong et al./ Sci. Total Environ. 145 (1994) 235-242
sions from cars travelling on motorways in the Netherlands, particular attention was focused on analysing the composition of traffic flows with respect to the vehicle parameters that are most likely to affect noise levels. As the noise emissions from cars travelling on motorways are primarily a function of the contact noise generated between tyres and the road surface, it is generally accepted that cars with wider tyres produce more noise because of the increased contact area. Since heavier cars tend to have tyres of greater width, it is normally assumed that the unladen weight of a car can be taken as an indirect measure of the car's noise emission potential. In Fig. 3, the breakdown of the national car fleet in terms of unladen weight is compared on the basis of ownership and use. Unfortunately, no data are available regarding the make-up of the various weight categories in terms of fuel types. Fig. 3 shows that heavier cars tend to be used more frequently than lighter cars. The largest proportion of kms travelled on the roads by any one weight category is 23%, i.e. in the case of cars
a central horizontal dash has been used to signify the mean value, and two horizontal dashes linked by vertical lines signifying the extent of the relevant confidence bands. Fig. 2 shows that only the composition measured at location 52 (AI3 motorway near Delft-Zuid) conforms to that predicted on the basis of average vehicle usage data compiled for the nation as a whole. In all other cases, significant differences were found in respect of either two or three of the fuel types under investigation, as evidenced by the fact that the 95% confidence intervals on the measured data do not encompass the figures on average car use in the Netherlands shown in Fig. 2. This finding is important since noise and pollutant emissions from LPG/petrol driven cars are likely to be significantly different from those associated with diesel engined cars. 3.2. Traffic flow unladen weight
composition
with respect
to
In view of the Road and Hydraulic Engineering Division's interest in measuring the noise emis-
N u m b e r o f p e t r o l e n g i n e d c a r s r e c o r d e d by t h e 4Q
Traffic
Engineering Department,
A u g u s t 1991
35
30
25
20
"E
T
{
{
15
10
<= ?
-750 I ?
13
751-850
52 21
? ,56
17 13
85'1-950 $2
21
] 56
17 13
951-1050 52
21
? 56
17 13
> 52
21
7 56
1? 13
Unladen
1050 52 21
L O C ~ t ~on 56
Fig. 4. Comparison of the composition of traffic flows at the six monitoring stations for petrol engined cars.
wePght
240
A.H. Versluis, M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
resulting traffic flow compositions with respect to both the location of the monitoring station (Fig. 4) and the time of day at which the data were collected (Fig. 5). It should be noted that any differences in the percentages quoted in Figs. 4 and 5 can only be considered to be significant if the confidence bands do not overlap. Reference to the figures shows that the extent of the divergence is greatest in the case of the heaviest and lightest weight categories. By comparison, no significant differences were found with respect to the proportion of petrol driven cars having unladen weights between 850 and 1050 kg in relation to either the location or the time of day at which the data were collected. In the case of diesel engine cars, clear differences were observed with respect to the composition of the various weight categories at different times of the day. However, these were only judged to be statistically significant in the case of the lightest and heaviest weight categories. In contrast, no significant differences were identified for diesel engine cars having weights between 1050 and 1250
weighing between 850 and 950 kg. By comparison, cars weighing between 750 and 850 kg account for a greater proportion of the national car fleet in terms of vehicle registrations. Statistical analyses of the data collected at the various monitoring stations have shown that differences in the composition of traffic flows with regard to place and time can be partially accounted for by differences in the type of fuel used. This follows on from the results quoted in the previous section and the fact that diesel powered cars are generally much heavier than cars with petrol engines. Further analyses were performed to investigate whether differences in the make-up of the various weight categories were identifiable as a function of place and time for cars using a given type of fuel. Separate analyses were therefore made of the information collected on petrol, diesel and LPG powered cars, in which the data recorded at the different locations and time frames were compared. Figs. 4 and 5 show the results of this comparison in the case of petrol driven cars. It can be seen that there are significant differences in the
Number of p e t r o l engined cars r e c o r d e d by the 40
T r a f f i c Engineering D e p a r t m e n t , August 1991 35
30
25
i
}
i
20
i I
15
i
i
I
v
! i
10
I i i i i
750
<=
mrrl
(:It
erh
251
mrl3
- 850
dt
ern
851-
rnrh
950
dt
erh
951-
rnrn
1050
Ot
erh
•
mrh
1050
Unladenwegnt
Ol
Period
ern
Fig. 5. Comparison of the composition of traffic flows at the six monitoring stations for petrol engined cars. (mrh, morning rush hour; dt, day time; erh, evening rush hour).
A.H. Versluis, M.E. van den Bol-de Jong et a l . / Sci. Total Environ. 145 (1994) 235-242
kg, neither were significant differences identified between the data collected at the various monitoring stations. By comparison, differences in the proportion of LPG powered vehicles in the observed traffic flows were even found to be statistically significant for various combinations of location and monitoring interval. However, as no distinct trends could be established, it was not possible to identify the specific weight categories which gave rise to these differences.
3.3. Traffic flow composition with respect to vehicle age As the noise emitted from cars travelling on motorways is principally derived from contact noise generated between tyres and the road surface (see Section 3.2), it is reasonable to assume that noise levels will also be a function of a vehicle's age - - since the tyres on older cars tend to be more worn. A further factor of importance in this context is the current popularity of heavier cars amongst the car buying public. As such vehicles tend to have wider tyres this could have implications for the scale of noise emission levels. Analysis of the composition of the national car fleet in terms of both ownership and use [3,4] shows that cars of up to 5 or 6 years old tend to be more intensively used than their older counterparts. It is also apparent that cars that are < 3 years old are used most frequently. To facilitate comparisons, similar statistical analyses have been made in relation to the age of the petrol, diesel and L P G powered vehicles passing the monitoring stations as discussed in Section 3.2. In the case of petrol driven cars, significant differences were identified in the composition of the traffic flows with respect to both the location and the time at which the data were collected. The most marked differences were discernible in the proportion of old cars built in or before 1983 and the proportion of new cars (1990-1991). It was also noticed that the percentage of cars built in or before 1983 was significantly lower in the morning rush hour than that recorded during the day and in the evening rush hour. By comparison, the data collected on diesel and L P G driven cars showed that there were even significant differences be-
241
tween combinations of locations and times of the day. Not withstanding this, detailed comparisons of the data collected at the various locations during the three discrete time frames have shown that the most significant differences were discernible with respect to the oldest groups of diesel and L P G vehicles (built in or before 1985) and with respect to the newest diesel and L P G cars (1990-1991). The results of these analyses have obvious parallels with the data discussed previously on petrol engined cars. Comparison of the data collected on the breakdown of traffic flows in terms of the age of vehicles has shown that at none of the locations and time frames was there evidence of an average composition for petrol engined cars as predicted on the basis of national statistics on car use [3,4]. However, the proportions of diesel and L P G powered cars of a given age did comply with the breakdowns predicted using national car use statistics at certain locations and time frames. Nevertheless, in spite of these differences, the resuits obtained for petrol engined cars were broadly similar to those for diesel and L P G powered vehicles. In general it was noted that the oldest petrol, diesel and LPG powered cars were consistently underrepresented relative to national usage figures, while at the same time, the proportion of relatively new vehicles was significantly higher than would be expected on the basis of these figures. This suggests that government statistics on car use tend to overestimate the use of older vehicles and underestimate the use of new cars.
3.4. The effect of differences in unladen weight on noise emissions In order to estimate the significance of differences in the composition of traffic flows for noise emission levels, an analysis was made of the unladen weight and predicted noise emissions of an average car passing the selected monitoring stations at the various time frames at which data were collected. For the purpose of this analysis, it was assumed that heavier cars have wider tyres and produce more contact noise than lighter vehicles (see Section 3.2). This was expressed in terms of a linear relationship varying from a noise level of 67.5 dB(A) for vehicles with an unladen weight of
242
A.H. Versluis, M.E. van den Bol-de Jong et a l . / Sci. Total Environ. 145 (1994) 235-242
675 kg and a tyre width of 135 mm, to a noise level of 71.5 dB(A) for cars with an unladen weight of 1350 kg and tyres measuring 225 mm in width (rolling noise, 60 km/h Standard 195/60R14 car) [51. In the present study the heaviest cars were observed to travel on the A13 motorway during the day at location 52 (Delft-Zuid), while the lightest cars were recorded at location 56 on the A29 motorway (Heinenoord) during the evening rush hour. The maximum difference in unladen weight therefore amounted to 120 kg. Similarly, the highest noise level was found to be associated with location 52 (Delft-Zuid) for traffic travelling during the day and the lowest noise level was attributed to location 56 on the A59 motorway (Heinenoord) for cars traveling during the evening rush hour. The maximum difference in noise levels as a result of differences in unladen weight was therefore estimated to be 0.8 dB(A). It was also calculated that differences of up to 0.5 dB(A) could be expected in the noise levels at different locations, but at the same time of day, while the maximum variation in noise levels at the same location - - but at different times of the day - - was estimated to be 0.3 dB(A). 4. Discussion
In the present study, it has been shown that national statistics on car ownership do not provide a satisfactory basis for assessing the make-up of the national used car fleet when measuring noise and pollution levels. Although national statistics on vehicle usage were found to be more representative, it is recommended that, for precise assessments, the composition of traffic flows be monitored in-situ. After comparing the composition of traffic flows at the six selected monitoring stations with the breakdown predicted on the basis of government statistics on average car use in the Netherlands, it was concluded that the breakdown in terms of fuel type only matched that predicted with the official car usage statistics at one of the six locations. The composition of the traffic flow at the other five locations was found to be significantly different from the breakdown predicted using average car use figures.
Significant differences in the composition of the traffic flows were also found with respect to the unladen weight and age of the vehicles passing the monitoring stations. A substantial part of these discrepancies was attributed to differences with respect to the type of fuel used by the vehicles passing the six monitoring stations at the times the data were recorded. Moreover, subsequent analyses showed that the residual differences with respect to unladen weight and age of the vehicles passing the monitoring sites were also statistically significant. It was concluded that these differences in traffic flow composition were a function of both the location and time at which the data were collected. It is therefore evident that the findings of the present study bring into question the validity of the assumption that, provided that the sample size is sufficiently large, the composition of traffic flows is the same throughout the Netherlands. It is clear that the composition of traffic flows can be both a function of place and time. Finally, it is concluded that special care must be taken in interpreting the composition of traffic flows when monitoring noise emission levels. Analyses have shown that differences of up to 0.8 dB(A) can be expected as a result of compositional differences with respect to only one key parameter: the unladen weight of the vehicles.
5. References 1
M.E. van den Bol-de-Jong and A.H. Versluis, Verschillen in samenstelling Nederlands personenautopark, TPDTNO-TUD and DWW-RWS, Delft, 1993. 2 Tweede Structuurschema Verkeer en Vervoer, Deel d regeringsbeslissing, Vergaderjaar 1989-1990, 20922, Nos. 15-16. 3 Centraal Bureau voor de Statistiek, Statistiek van de motorvoertuigen, Voorburg/Heerlen, 1991. 4 Centraal Bureau voor de Statistiek, Het bezit en gebruik van personenauto's 1991, Voorburg/Heerlen, 1992. 5 Continental Aktiengesellschaft. 6 S.E. Fienberg, The Analysis of Cross-Classified Categorical Data, M.I.T, Massachusetts, 1979. 7 A.J. Dobson, An Introduction to Generalized Linear Models, Chapman & Hall, London, 1990. 8 J.M.M. Bishop, S.E. Fienberg and P.W. Holland, Discrete Multivariate Analysis: Theory and Practice, M.I.T. Massachusetts, 1975.
The Science of the Total Environment 145 (1994) 235-242
Further study of the composition of the Dutch car fleet ..a
A . H . Versluls
, M.E. van den Bol-de J o n g b
aRoad and Hydraulic Engineering Division. Directorate General for Public Works and Water Management, P.O. Box 5044, 2600 GA Delft, The Netherlands hCentre for Applied Statistics, TNO Institute of Applied Physics, P.O. Box 155, 2600 AD Delft, The Netherlands
(Received 4 February 1993; accepted 2 March 1993)
Abstract This article discusses the results of an investigation into the composition of the Dutch car fleet with regard to salient vehicle characteristics such as fuel type, unladen weight and age. Data for this study were collected by recording the registration numbers of cars passing six motorway locations at three distinct times of the day. Analyses of the results have revealed significant differences in the composition of these traffic flows with respect to both place and time. It is therefore recommended that account be taken of such differences when interpreting the results of noise emission measurements and measurements of air-borne pollutants and soil contaminants emitted by passing vehicles. It has been shown, for instance, that variations in the composition of traffic flows with respect to the unladen weight of the constituent vehicles could give rise to differences in noise emissions of up to 0.8 dB(A) for the average individual car between locations and times of the day. However, it should be noted that the unladen weight of a vehicle is only one of a number of characteristics that can affect noise levels. Speed, fuel type, engine size, tyre tread and the age of vehicles are all known to influence the level of noise generated when a car travels on the road. It therefore seems reasonable to assume that any differences in the composition of traffic flows with respect to these parameters will produce even larger variations in in-situ noise emission levels than are quoted in this paper. Key words." Noise emissions; Fuel type; Passenger car fleet; Unladen weight; Vehicle age
1. Introduction The D i r e c t o r a t e G e n e r a l for Public W o r k s and W a t e r M a n a g e m e n t in the N e t h e r l a n d s regularly m o n i t o r s traffic flow on D u t c h r o a d s in o r d e r to quantify traffic noise and to determine the concentration o f air b o r n e pollutants and soil contaminants emitted by passing vehicles. It is k n o w n * Corresponding author.
that emissions o f this type can v a r y from vehicle to vehicle, which implies that noise a n d p o l l u t a n t emissions are not only d e p e n d e n t on traffic density but also on the c o m p o s i t i o n o f the traffic flow. Until now little account has been taken o f possible differences in the m a k e - u p o f traffic flows. A p a r t from a limited n u m b e r o f studies, in which consideration was given to the volume o f heavy g o o d s vehicles on the road, it has generally been assumed that a given sample could be regarded as being
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(93)03687-W
236
A.H. Versluis. M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
representative of the average car fleet in the country, provided that the traffic density was of a sufficient level at the time the measurements were made. In practice, this has meant that differences in the composition of traffic flows at various monitoring stations and times of the day have been considered to be negligible provided that the samples taken were sufficiently large. It has been recognised that considerable differences exist between the composition of national car fleets. For instance, the composition of local traffic flows in Poland and the USA is likely to differ. If such differences are apparent on an international level, it therefore seems reasonable to question assumptions about the homogeneity of traffic flows within a single country. Key vehicle characteristics that are important in this context are: fuel type, engine size, vehicle age, speed, unladen weight, tyre width and tyre tread. It was therefore decided to investigate the correctness of the assumption that the composition of traffic flows over a certain size are broadly similar throughout the Netherlands, by recording the unladen weight, fuel type and age of the vehicles passing selected monitoring stations [1]. Where possible, comparisons were made with data obtained from the national vehicles register kept by the Government Road Transport Department. For the used fleet, use was also made of information compiled by the Central Bureau of Statistics concerning the average number of kilometres travelled on Dutch roads each year by particular types of car.
2. Monitoring procedure At the end of August 1991, the Traffic Engineering Department of the Directorate General for Public Works and Water Management recorded the registration numbers of cars passing six monitoring stations along various motorways in the Netherlands. The precise location of the monitoring stations at which the visual registrations took place is given in Table 1. In choosing the six sites, a deliberate effort was made to select two examples of each of the three main types of motorway encountered in the Netherlands. Consideration was also given to maintaining an acceptable distance between the various monitoring stations and ensuring that a sufficient geographic spread was achieved. The number plates of passing cars were noted at each of the six locations 3 times/day. This involved recording vehicle registration numbers for 2 h during the morning and evening rush hours, and for a 4-h period during the day. Each registration period was divided into a series of 15-min intervals. The first 70 cars were sampled during each of the eight intervals in the two rush hour monitoring sessions, and the first 35 cars during the 16 intervals in the daytime monitoring sessions. This allowed at least 500 registration numbers to be recorded during each monitoring session. It was decided to collect such a large number of registration numbers in order to ensure that a sufficiently representative sample was obtained.
Table 1 Monitoring stations at which the registration numbers of passing vehicleswere recorded Location number
Motorway
Town
Type of roada
7 13 17 21 52 56
A59 A59 A58 A50 A 13 A29
Made Vlijmen Moergestel Ravenstein Delft-Zuid Heinenoord
Link to main road network Link to main road network Hinterland connection Main transport route Hinterland connection Main transport route
"Classification in accordancewith Ref. [2].
A.H. Versluis, M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
237
Ownership and Use of Passenger Cars Total
Dutch
car
fleet in
August
1991
80
70
60
car
ownership
use
[kin/year]
[vhc]
!
~-~ ~--~ 5 0 ~D
~
4O Total car fleet 5.244 million cars Total annual use 84.16 million km
IlJ
2O
10 Diesel Tuel type
Petrol
LPG
Fig. 1. Compostition of the Dutch car fleet based on vehicle registration data for August 1991 and on statistics regarding the average use of vehicles on Dutch roads.
Number
of csrs reco:ded
80
by the Traffic
August
Engmeer:ng
De~ar-Lmen[
1991
90
t
60
513
40
& 30
20
10
Petrol 7
13
19
21
O l e 519 I 52
56
?
13
17
21
52
L PC~ 56
?
13
in
21
~uel 52
56
type
t_o~-a b a n
Fig. 2. Comparison of the composition of traffic flows at the six monitoring stations with respect to fuel type and in relation to the average use of cars on Dutch roads.
238
A.H. Versluis, M.E. van den Bol-de Jong et al./Sci. Total Environ. 145 (1994) 235-242
3. Results 3.1. Traffic f l o w composition with respect to f u e l type
It is important to note that when discussing traffic flow measurements, a distinction should be made between the composition of the national car fleet as based on vehicle registrations and the composition obtained from data on vehicle use. For the purpose of practical environmental research, measurements are normally made of actual traffic flows, which should reflect statistics on vehicle usage. Fig. 1. illustrates the considerable differences that exist between the composition of the national car fleet in terms of ownership and the use of particular types of vehicle in the Netherlands. The data shown in Fig. 1 contrast the average distance travelled in petrol, diesel and liquid peteroleum gas (LPG) powered cars with the number of such vehicles registered in the Netherlands in August 1991. For comparison purposes, both distributions are expressed on a percentage basis. Fig. 1 shows that the breakdown of the national car fleet in terms of ownership is different from the
composition obtained on the basis of vehicle use and that diesel and L P G powered cars are used more frequently than petrol driven cars. In the present study, the registration numbers of - 500 cars were recorded at the six selected monitoring stations at three different times of the day. The composition of the vehicle registration numbers was analysed statistically using log-linear models [6-8]. It was shown that while the composition of the traffic flow with regard to fuel type did not differ significantly at various times of the day, significant differences were found with regard to the make-up of the traffic flow passing the six selected monitoring stations. The breakdown of the traffic flow with regard to fuel type for the six locations is shown in Fig. 2, together with the composition calculated on the basis of average vehicle usage figures compiled with data from the Government Road Transport Department and the Central Bureau of Statistics (horizontal line) [3,4]. The data on the type of fuel used in the cars monitored at the different locations have been expressed in terms of a mean value and associated 95% confidence levels. In each case,
Ownership and Use of Passenger Cars Total Dutch car
f l e e t in A u g u s t
1991
25 car ownership [vhc] I '+ use [krn/year ]
20
C
~1o Total car fleet ,5.244 million cars Total annual use 84.16 million km
13..
1 tO0 1300 1000 1200 >1350 Unladen Weight [kg] 9OO
<7 800
Fig. 3. Composition of the national Dutch car fleet categorised according to unladen weight which is based on ownership data for August 1991 and average usage in 1991.
239
A.H. Versluis, M.E. van den Bol-de Jong et al./ Sci. Total Environ. 145 (1994) 235-242
sions from cars travelling on motorways in the Netherlands, particular attention was focused on analysing the composition of traffic flows with respect to the vehicle parameters that are most likely to affect noise levels. As the noise emissions from cars travelling on motorways are primarily a function of the contact noise generated between tyres and the road surface, it is generally accepted that cars with wider tyres produce more noise because of the increased contact area. Since heavier cars tend to have tyres of greater width, it is normally assumed that the unladen weight of a car can be taken as an indirect measure of the car's noise emission potential. In Fig. 3, the breakdown of the national car fleet in terms of unladen weight is compared on the basis of ownership and use. Unfortunately, no data are available regarding the make-up of the various weight categories in terms of fuel types. Fig. 3 shows that heavier cars tend to be used more frequently than lighter cars. The largest proportion of kms travelled on the roads by any one weight category is 23%, i.e. in the case of cars
a central horizontal dash has been used to signify the mean value, and two horizontal dashes linked by vertical lines signifying the extent of the relevant confidence bands. Fig. 2 shows that only the composition measured at location 52 (AI3 motorway near Delft-Zuid) conforms to that predicted on the basis of average vehicle usage data compiled for the nation as a whole. In all other cases, significant differences were found in respect of either two or three of the fuel types under investigation, as evidenced by the fact that the 95% confidence intervals on the measured data do not encompass the figures on average car use in the Netherlands shown in Fig. 2. This finding is important since noise and pollutant emissions from LPG/petrol driven cars are likely to be significantly different from those associated with diesel engined cars. 3.2. Traffic flow unladen weight
composition
with respect
to
In view of the Road and Hydraulic Engineering Division's interest in measuring the noise emis-
N u m b e r o f p e t r o l e n g i n e d c a r s r e c o r d e d by t h e 4Q
Traffic
Engineering Department,
A u g u s t 1991
35
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25
20
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T
{
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Fig. 4. Comparison of the composition of traffic flows at the six monitoring stations for petrol engined cars.
wePght
240
A.H. Versluis, M.E. van den Bol-de .long et al. / Sci. Total Environ. 145 (1994) 235-242
resulting traffic flow compositions with respect to both the location of the monitoring station (Fig. 4) and the time of day at which the data were collected (Fig. 5). It should be noted that any differences in the percentages quoted in Figs. 4 and 5 can only be considered to be significant if the confidence bands do not overlap. Reference to the figures shows that the extent of the divergence is greatest in the case of the heaviest and lightest weight categories. By comparison, no significant differences were found with respect to the proportion of petrol driven cars having unladen weights between 850 and 1050 kg in relation to either the location or the time of day at which the data were collected. In the case of diesel engine cars, clear differences were observed with respect to the composition of the various weight categories at different times of the day. However, these were only judged to be statistically significant in the case of the lightest and heaviest weight categories. In contrast, no significant differences were identified for diesel engine cars having weights between 1050 and 1250
weighing between 850 and 950 kg. By comparison, cars weighing between 750 and 850 kg account for a greater proportion of the national car fleet in terms of vehicle registrations. Statistical analyses of the data collected at the various monitoring stations have shown that differences in the composition of traffic flows with regard to place and time can be partially accounted for by differences in the type of fuel used. This follows on from the results quoted in the previous section and the fact that diesel powered cars are generally much heavier than cars with petrol engines. Further analyses were performed to investigate whether differences in the make-up of the various weight categories were identifiable as a function of place and time for cars using a given type of fuel. Separate analyses were therefore made of the information collected on petrol, diesel and LPG powered cars, in which the data recorded at the different locations and time frames were compared. Figs. 4 and 5 show the results of this comparison in the case of petrol driven cars. It can be seen that there are significant differences in the
Number of p e t r o l engined cars r e c o r d e d by the 40
T r a f f i c Engineering D e p a r t m e n t , August 1991 35
30
25
i
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1050
Ot
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Ol
Period
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Fig. 5. Comparison of the composition of traffic flows at the six monitoring stations for petrol engined cars. (mrh, morning rush hour; dt, day time; erh, evening rush hour).
A.H. Versluis, M.E. van den Bol-de Jong et a l . / Sci. Total Environ. 145 (1994) 235-242
kg, neither were significant differences identified between the data collected at the various monitoring stations. By comparison, differences in the proportion of LPG powered vehicles in the observed traffic flows were even found to be statistically significant for various combinations of location and monitoring interval. However, as no distinct trends could be established, it was not possible to identify the specific weight categories which gave rise to these differences.
3.3. Traffic flow composition with respect to vehicle age As the noise emitted from cars travelling on motorways is principally derived from contact noise generated between tyres and the road surface (see Section 3.2), it is reasonable to assume that noise levels will also be a function of a vehicle's age - - since the tyres on older cars tend to be more worn. A further factor of importance in this context is the current popularity of heavier cars amongst the car buying public. As such vehicles tend to have wider tyres this could have implications for the scale of noise emission levels. Analysis of the composition of the national car fleet in terms of both ownership and use [3,4] shows that cars of up to 5 or 6 years old tend to be more intensively used than their older counterparts. It is also apparent that cars that are < 3 years old are used most frequently. To facilitate comparisons, similar statistical analyses have been made in relation to the age of the petrol, diesel and L P G powered vehicles passing the monitoring stations as discussed in Section 3.2. In the case of petrol driven cars, significant differences were identified in the composition of the traffic flows with respect to both the location and the time at which the data were collected. The most marked differences were discernible in the proportion of old cars built in or before 1983 and the proportion of new cars (1990-1991). It was also noticed that the percentage of cars built in or before 1983 was significantly lower in the morning rush hour than that recorded during the day and in the evening rush hour. By comparison, the data collected on diesel and L P G driven cars showed that there were even significant differences be-
241
tween combinations of locations and times of the day. Not withstanding this, detailed comparisons of the data collected at the various locations during the three discrete time frames have shown that the most significant differences were discernible with respect to the oldest groups of diesel and L P G vehicles (built in or before 1985) and with respect to the newest diesel and L P G cars (1990-1991). The results of these analyses have obvious parallels with the data discussed previously on petrol engined cars. Comparison of the data collected on the breakdown of traffic flows in terms of the age of vehicles has shown that at none of the locations and time frames was there evidence of an average composition for petrol engined cars as predicted on the basis of national statistics on car use [3,4]. However, the proportions of diesel and L P G powered cars of a given age did comply with the breakdowns predicted using national car use statistics at certain locations and time frames. Nevertheless, in spite of these differences, the resuits obtained for petrol engined cars were broadly similar to those for diesel and L P G powered vehicles. In general it was noted that the oldest petrol, diesel and LPG powered cars were consistently underrepresented relative to national usage figures, while at the same time, the proportion of relatively new vehicles was significantly higher than would be expected on the basis of these figures. This suggests that government statistics on car use tend to overestimate the use of older vehicles and underestimate the use of new cars.
3.4. The effect of differences in unladen weight on noise emissions In order to estimate the significance of differences in the composition of traffic flows for noise emission levels, an analysis was made of the unladen weight and predicted noise emissions of an average car passing the selected monitoring stations at the various time frames at which data were collected. For the purpose of this analysis, it was assumed that heavier cars have wider tyres and produce more contact noise than lighter vehicles (see Section 3.2). This was expressed in terms of a linear relationship varying from a noise level of 67.5 dB(A) for vehicles with an unladen weight of
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A.H. Versluis, M.E. van den Bol-de Jong et a l . / Sci. Total Environ. 145 (1994) 235-242
675 kg and a tyre width of 135 mm, to a noise level of 71.5 dB(A) for cars with an unladen weight of 1350 kg and tyres measuring 225 mm in width (rolling noise, 60 km/h Standard 195/60R14 car) [51. In the present study the heaviest cars were observed to travel on the A13 motorway during the day at location 52 (Delft-Zuid), while the lightest cars were recorded at location 56 on the A29 motorway (Heinenoord) during the evening rush hour. The maximum difference in unladen weight therefore amounted to 120 kg. Similarly, the highest noise level was found to be associated with location 52 (Delft-Zuid) for traffic travelling during the day and the lowest noise level was attributed to location 56 on the A59 motorway (Heinenoord) for cars traveling during the evening rush hour. The maximum difference in noise levels as a result of differences in unladen weight was therefore estimated to be 0.8 dB(A). It was also calculated that differences of up to 0.5 dB(A) could be expected in the noise levels at different locations, but at the same time of day, while the maximum variation in noise levels at the same location - - but at different times of the day - - was estimated to be 0.3 dB(A). 4. Discussion
In the present study, it has been shown that national statistics on car ownership do not provide a satisfactory basis for assessing the make-up of the national used car fleet when measuring noise and pollution levels. Although national statistics on vehicle usage were found to be more representative, it is recommended that, for precise assessments, the composition of traffic flows be monitored in-situ. After comparing the composition of traffic flows at the six selected monitoring stations with the breakdown predicted on the basis of government statistics on average car use in the Netherlands, it was concluded that the breakdown in terms of fuel type only matched that predicted with the official car usage statistics at one of the six locations. The composition of the traffic flow at the other five locations was found to be significantly different from the breakdown predicted using average car use figures.
Significant differences in the composition of the traffic flows were also found with respect to the unladen weight and age of the vehicles passing the monitoring stations. A substantial part of these discrepancies was attributed to differences with respect to the type of fuel used by the vehicles passing the six monitoring stations at the times the data were recorded. Moreover, subsequent analyses showed that the residual differences with respect to unladen weight and age of the vehicles passing the monitoring sites were also statistically significant. It was concluded that these differences in traffic flow composition were a function of both the location and time at which the data were collected. It is therefore evident that the findings of the present study bring into question the validity of the assumption that, provided that the sample size is sufficiently large, the composition of traffic flows is the same throughout the Netherlands. It is clear that the composition of traffic flows can be both a function of place and time. Finally, it is concluded that special care must be taken in interpreting the composition of traffic flows when monitoring noise emission levels. Analyses have shown that differences of up to 0.8 dB(A) can be expected as a result of compositional differences with respect to only one key parameter: the unladen weight of the vehicles.
5. References 1
M.E. van den Bol-de-Jong and A.H. Versluis, Verschillen in samenstelling Nederlands personenautopark, TPDTNO-TUD and DWW-RWS, Delft, 1993. 2 Tweede Structuurschema Verkeer en Vervoer, Deel d regeringsbeslissing, Vergaderjaar 1989-1990, 20922, Nos. 15-16. 3 Centraal Bureau voor de Statistiek, Statistiek van de motorvoertuigen, Voorburg/Heerlen, 1991. 4 Centraal Bureau voor de Statistiek, Het bezit en gebruik van personenauto's 1991, Voorburg/Heerlen, 1992. 5 Continental Aktiengesellschaft. 6 S.E. Fienberg, The Analysis of Cross-Classified Categorical Data, M.I.T, Massachusetts, 1979. 7 A.J. Dobson, An Introduction to Generalized Linear Models, Chapman & Hall, London, 1990. 8 J.M.M. Bishop, S.E. Fienberg and P.W. Holland, Discrete Multivariate Analysis: Theory and Practice, M.I.T. Massachusetts, 1975.