The effects of travelling speed on road safety in urban areas have been investigated in two studies conducted by the Road Accident Research Unit of the University of Adelaide. One of these studies examined the relationship between the travelling speed of a passenger car in a 60 km/h speed limit area and the risk of involvement in a casualty crash. The other study was based on detailed investigations of fatal pedestrian collisions. An assessment was made of the likely consequences of changes in the travelling speeds of the vehicles involved.
Travelling Speed and the Relative Risk of Crash Involvement
Investigators from the Road Accident Research Unit attended the scene of 952 crashes to which an ambulance was called. From these 952 crashes, 148 crashes and 151 case vehicles met the criteria for inclusion in the study.
These case vehicles were passenger cars which had been moving at a free travelling speed (ie not slowing to leave - or accelerating into - the traffic stream) before being involved in a crash in which at least one person was injured seriously enough to be transported by ambulance to hospital. The drivers of case vehicles were also required to have a zero blood alcohol concentration, to exclude the effects of alcohol on the risk of crashing.
The impact speeds were reconstructed using the latest computer aided crash reconstruction techniques and other evidence (such as skid marks due to pre-impact braking) was used to estimate the speed lost, if any, before the impact.
The 604 control vehicles (four per case) were passenger cars matched to the cases by location of the crash, direction of travel, time of day, and day of week. Their speeds were measured with a laser speed meter.
The risk of crash involvement was expressed as a risk relative to that experienced by a driver travelling at 60 km/h. It can be seen in Figure 1 that the relative risk of crash involvement approximately doubles for each increase of 5 km/h in travelling speed above the 60 km/h speed limit.
By working back from the risk estimates, we estimate that nearly half (46 per cent) of these casualty crashes probably would have been avoided, or reduced to non-casualty crashes, if none of the case vehicles had been travelling above the 60 km/h speed limit.
A more conservative estimate, based on calculation of stopping distances and impact speeds, indicates that 29 per cent of crashes would have been avoided altogether, with an overall reduction of 22 per cent in the impact energy of the remaining cases.
Using the second, more conservative, method we also estimate that a 10 km/h reduction in the travelling speeds of the crash involved cars in this study would probably have resulted in a reduction of at least 42 per cent in the number of crashes.
A 5 km/h reduction showed much less effect but would still have resulted in a reduction of at least 15 per cent in the number of crashes.
Again using the conservative method, we estimate that an urban area speed limit of 50 km/h on all roads, with the present level of compliance, would be likely to result in a reduction of at least 33 per cent in the number of free travelling speed casualty crashes.
As shown in the following table, the great majority of these free travelling speed crashes occurred on arterial roads or major traffic routes, Therefore, a speed limit of 50 km/h in local streets would be likely to have only a small effect on free travelling speed casualty crashes as a whole (a 6% reduction) due mainly to the small proportion of these crashes which occurred on local streets.
Type of Road by Number of Crashes in the Speed Case Control Study
| Road Type |
Crashes |
|
Number |
Per cent |
Major traffic route |
127 |
85.8 |
Local street |
21 |
14.2 |
Total |
148 |
100.0 |
Vehicle Travelling Speeds and Pedestrian Fatalities
One hundred and fifty three fatal pedestrian collisions that occurred in the 60 km/h speed limit area of metropolitan Adelaide were investigated by the Road Accident Research Unit. The investigation of each case commenced with attendance at the autopsy of a fatally injured pedestrian and continued with an examination of the vehicle involved and the scene of the collision. In most cases, statements were available from the driver and from any witnesses.
In 133 of these 153 collisions with a pedestrian, the striking vehicle had a free travelling speed. The other 20 cases involved vehicles that were turning, accelerating from a standstill, or had run off the carriageway, and some cases where it was clear that the pedestrian had intended to commit suicide or the driver had collapsed at the wheel.
In 15 of the 133 free travelling speed cases, there was not sufficient information available to estimate that speed reliably and so the calculations of the effects of changes to travelling speeds were based on 118 fatal pedestrian collisions. The estimated effects of the hypothetical changes to the travelling speeds of these 118 cases were then related back to the full sample of 153 cases to give an estimate of the overall effect on the incidence fatal pedestrian collisions.
In 45 per cent of the 118 cases which could be analysed, the driver stated that no evasive action was attempted prior to the collision - typically because the pedestrian was not seen at all or the driver did not realise there was danger of a collision. So the impact speed in these cases was equal to the travelling speed of the vehicle.
In the other 55 per cent of the analysed cases, some evasive action was taken. As the evasive action was typically emergency braking, the travelling speed of the vehicle could usually be estimated from braking skid marks. A description of the crash reconstruction techniques used is given in the full report on this study [McLean et al, 1994].
The distance the vehicle travelled during the driver's reaction time and, where relevant, the length of the skid mark before the impact, gave the position of the vehicle relative to the point of impact when the driver realised that a collision was likely. The reaction time was assumed to be 1.5 seconds but this was modified in certain cases (eg an intoxicated driver).
The likely effect of the reduced travelling speeds on the number of fatalities was based on an estimate of the probability of the pedestrian being fatally injured at a given impact speed. This estimate was based on the best available data from a number of sources. At an impact speed of 40 km/h the probability of the pedestrian being fatally injured was estimated to be 25 per cent. This probability increased rapidly with increasing impact speed; by 50 km/h it was estimated to be to 85 per cent.
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So why not 25 km/h, then?
We posed the following question to one of the report's authors, Craig Kloeden. Following the arguments contained in this paper, can't an even stronger case be made for an urban speed limit of, say, 25 km/h? Here's what he had to say: We did not directly address that issue in our study as we did not have vehicles travelling at that speed. However, the basic physical principles and common sense both show that slowing vehicles down will reduce the incidence and severity of crashes. We stress going to 50 km/h in our jurisdiction as we currently have a 60 km/h limit. There are European countries that have 30 km/h limits in built-up areas. The other side of the argument is that you get diminishing returns from ever lower speed limits and increased travel times as well as the political cost of lowering speed limits. So yes, very slow traffic is good from a safety perspective but ultimately other balancing factors mean that the optimal level is higher than the optimal safety level (zero?).
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As noted above, the estimated percentage reductions in fatal pedestrian collisions are calculated for the total sample of 153 cases. In other words, the magnitude of the effect on pedestrian fatalities of each listed change in travelling speeds has been reduced to allow for those pedestrian collisions which would not be expected to be affected by a change in the travelling speed of the vehicle. The overall effects of the various speed change scenarios are shown in Figure 2.
A 10-km/h reduction in the travelling speed of all vehicles would almost halve the number of pedestrian fatalities (a reduction of 48%), including the elimination of 22 per cent of the collisions altogether (those vehicles would have stopped before hitting the pedestrian). Note that this scenario refers to a reduction of 10 km/h by all vehicles, not only those exceeding the speed limit.
If the travelling speed of all vehicles were reduced by 5 km/h there would be an estimated 32 per cent reduction in fatalities, including a 10 per cent reduction in the number of collisions. However, if the 5 km/h speed reduction applied only to vehicles on local streets, the effect on pedestrian fatalities would be very much less (a reduction of 4% in fatalities including 1% of collisions prevented). As in the previous study, few of the cases occurred in local streets (15% of the total).
If enforcement of the 60 km/h speed limit could be 100 per cent effective (ie without any enforcement tolerance) the reduction in pedestrian fatalities would be much less than for an across-the-board 5 km/h reduction in the travelling speed of all vehicles (13% and 32% respectively). Elimination of all vehicles travelling above 70 km/h (the speed limit plus a 10 km/h enforcement tolerance) would naturally be less effective again (a reduction of 8%). However, a change to an urban area speed limit of 50 km/h, with the same level of compliance as at present, would be expected to reduce fatal pedestrian collisions by 27 per cent, including the prevention of 12 per cent of the collisions.
Discussion
The risk of being seriously injured in a road crash is low. In Australia about one person in 1,000 is hospitalised per year for treatment of injuries sustained in a road crash, or about one in eleven per lifetime. Fatality rates from road crashes are about one tenth of those for hospitalisations. Nevertheless, a low risk affecting the whole population results in road crashes being one of the leading causes of death and disability.
Even though the average risk for an individual may be low, there can be very large relative differences in that risk. For example, we estimate that a driver travelling at 75 km/h in a 60 km/h speed limit area has a risk of being involved in a casualty crash that is 10 times greater than that of a driver travelling at 60 km/h.
Much has been written about the setting and enforcing of speed limits. This discussion does not attempt to review that literature but rather concentrates on the implications of the two studies described above.
Implications for Enforcement
It is customary for an enforcement tolerance to be added to the legal speed limit such that a driver is not charged with a speeding violation below a speed of about 70 km/h in a 60 km/h speed limit area. The reasons for this tolerance go back to the days when speed limits were enforced by a police vehicle following a speeding vehicle for a sufficient length of time for the police officer to be able to state that its speed was the same as that which was indicated on the speedometer of the police vehicle. With the introduction of radar speed meters, and more recently laser speed meters, the speed of a vehicle can be measured accurately to within a small fraction of 1 km/h.
It is also pointed out that practical policing requires some tolerance in enforcement. That is understandable and clearly desirable in many matters. However, the results of the study of travelling speed and the risk of involvement in a casualty crash show that the risk is twice as great at 65 km/h as it is at 60 km/h, and four times as great at 70 km/h. Proportional increases in risk of such magnitude would appear to be sufficient reason to justify the elimination of the current practice of applying an enforcement tolerance to speed limits, or at least a substantial reduction in such a tolerance.
In South Australia there is a zero tolerance approach to policing drink-driving legislation. We have shown in the full report on the speed case control study that travelling at 65 km/h in a 60 km/h speed limit area increases the risk of involvement in a casualty crash by the same amount as driving at the speed limit with a blood alcohol level of 0.05 (Kloeden et al, 1997). As all cars are fitted with speedometers, and few if any drivers have access to a breath alcohol meter, it is incongruous that zero tolerance should apply to drink driving but not to speeding.
There appears to be considerable public support for a reduction in the enforcement tolerance to speed limits. In a recent Australian national survey (Mitchell-Taverner et al, 1997) people were asked "Now thinking about 60 km/h speed zones in urban areas, how fast should people be allowed to drive without being booked for speeding?" The results showed that 44 per cent of people believed that 60 km/h limits should be strictly enforced. A further 34 per cent would tolerate exceeding the limit by 5 km/h and 18 per cent expressed the view that 70 km/h would be acceptable in current 60 km/h speed zones.
Although the risk of crash involvement increases rapidly with increasing speed, the overall contribution of speeding to crash causation is much greater at speeds below, say, 75 km/h than it might appear from the risk curve in Figure 1. In that study, more than two thirds of the crashes involving speeding cars occurred at a speed that was below 75 km/h, because many more drivers are travelling in the speed range from 61 to 74 km/h than above the latter speed.
Eliminating speeding vehicles which are travelling above the 60 km/h limit can be expected to be of greater benefit to car occupants than to pedestrians because few pedestrians could be expected to survive being struck by a vehicle at an impact speed above 60 km/h. In South Australia, from 1991 to 1997, 17 per cent of the vehicles involved in fatal pedestrian collisions in 60 km/h speed zones were travelling faster than 60 km/h (Kloeden et al, 1999). This is a similar percentage to the estimated reduction of 13 per cent if all vehicles travelling faster than 60 km/h were eliminated, as shown in Figure 2.
Implications for Setting the Urban Area Speed Limit
There is increasing acceptance of the desirability of reducing speed limits in residential areas and local streets. However, proposals for such a change are often accompanied by a statement that the 60 km/h speed limit on urban arterial roads will be retained or zoned upwards.
Eighty six per cent of the crashes on which the study of travelling speed and the risk of crash involvement was based occurred on arterial roads or alternate traffic routes and the remaining 14 per cent on local streets. These percentages are almost identical to the corresponding distribution of the cases in the study of the fatal pedestrian collisions. They emphasise the need to reduce speed limits on arterial roads as well as on local streets if safety is the main concern.
As noted above, we estimate that a general urban area speed limit of 50 km/h on all roads, with the present level of compliance, would be likely to result in a reduction of at least 33 per cent in the number of free travelling speed casualty crashes involving sober drivers. This does not mean a reduction of that magnitude in all casualty crashes because many of them are due to intoxicated drivers or involve manoeuvres which have been excluded from the cases selected for the study. Nevertheless, the results of this study indicate an opportunity to reduce substantially the frequency of casualty crashes in urban areas in Australia.
It is commonly accepted by traffic engineers that a speed limit should appear to be reasonable to drivers. Consequently, the 85th percentile speed of the traffic is often taken as a guide to the selection of the speed limit. That, in turn, can be used to justify retention of a 60 km/h speed limit on urban arterial roads. However, for that process to result in safe travelling speeds it must be assumed that drivers are capable of making a realistic assessment of the risk of crash involvement and drive accordingly.
Fortunately, very few drivers will ever be involved in a collision with a pedestrian. Therefore, the average driver cannot be expected to have a realistic understanding of how the risk of such a collision varies according to the speed at which they choose to travel on an urban arterial road.
It is unlikely that any driver realises that a difference of 10 km/h in travelling speed (60 km/h & 50 km/h) can result in the pedestrian being struck at 44 km/h or not at all (McLean et al, 1994).
The estimated reduction of 27 per cent in pedestrian fatalities if the urban area speed limit were to be reduced to 50 km/h on all roads is very similar to the reduction that did occur following such a change in Zurich, Switzerland. In the year following a change in the speed limit from 60 to 50 km/h there was a reduction of 16 per cent in pedestrian collisions and a reduction of 25 per cent in pedestrian fatalities [Walz, 1983].
It is relevant to note that Victoria and New South Wales both had an urban area speed limit of 30 mph, or 48 km/h. The limit was raised to 35 mph (56 km/h) in Victoria in 1963 and in NSW in 1964 on the grounds that roads and cars were much safer than when the 30 mph limit was introduced in the 1930s. When metrication replaced imperial measure in 1974 the traffic engineers of the day felt constrained to choose between 50 or 60 km/h, but not 55 km/h, for the metric equivalent of 56 km/h. (It was accepted traffic engineering practice at that time for speed limits to end in a zero and advisory speeds to end in a five.) They chose 60 km/h. That is why Australia has the highest urban area speed limit of any OECD country.
At a conservative estimate, about two thousand pedestrians have died in Australia since 1974 because 60 km/h was chosen rather than 50 km/h....
Acknowledgements
The speed case control study was funded by the Federal Office of Road Safety and Transport SA, with additional support from a Research Unit grant from the National Health and Medical Research Council.
The study of the effect of travelling speed on pedestrian fatalities was funded by the Federal Office of Road Safety and was based on data collected with the support provided by the National Health and Medical Research Council.
References
Kloeden CN, McLean AJ, Moore VM, Ponte G. 1997. Travelling speed and the risk of crash involvement. Canberra: Federal Office of Road Safety. Report CR 172. 72 p.
Kloeden CN, White K, McLean AJ. 1999. Characteristics of Fatal and Severe Pedestrian Accidents in South Australia. Adelaide: Road Accident Research Unit, University of Adelaide. Report to Transport SA (in draft).
McLean AJ, Anderson RWG, Farmer MJB, Lee BH and Brooks CG. 1994. Vehicle travel speeds and the incidence of fatal pedestrian collisions. Canberra: Federal Office of Road Safety: Report CR 146. 82p.
Mitchell-Taverner P, Adams K and Hejtmanek S. 1997. Community attitudes to road safety: Community attitudes survey wave 10. Canberra: Federal Office of Road Safety.
Transportation Research Board. 1999. Managing speed: Review of current practice for setting and enforcing speed limits. Washington, D.C.: National Academy Press. TRB Special Report 254. 427 p.
Walz F.H., Hoefliger M., Fehlmann W. 1983. 'Speed limit reduction from 60 to 50 km/h and pedestrian injuries'. Warrendale, Pennsylvania: Society of Automotive Engineers. Twenty-Seventh Stapp Car Crash Conference Proceedings with International Research Council on Biokinetics of Impacts (IRCOBI). pp. 311-318.
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