The relationships between enforcement effort and speeds
The relationships between enforcement effort and vehicle speeds and also vehicle speeds and crash risk.
Speed enforcement is designed to affect speeds via a deterrent effect on drivers. The effect of deterrence on the driver’s speed choice depends on:
- the perceived risk of being caught,
- the fear of being caught and
- the fear of the resulting punishment
The risk of a given driver being involved in a fatal crash was 35% lower during the month after receipt of a speeding ticket, than in a comparable month during which no speeding ticket was received. General deterrence targets the wider population by increasing the perception that offenders will be caught, by means both of police activity and publicity about this activity. (study in Canada, Redelheimer et al).
New Zealand (Mara et al)
Enforcement activity taken into account includes:
- speeding infringements (camera and non-camera),
- hidden and visible speed camera activity and
- the advent of marked State Highway Patrol cars.
Estimated reductions in open road mean speeds of 0.7% and 0.8% were found, associated with each increase of 10000 speed camera infringements and 10000 other speed infringements respectively.
Higher reductions of 1.1% and 1.6% were found in the 85th percentile speeds. An estimated injury crash reduction of 12% was found to be associated with a 1km/h reduction in mean open road speed.
Mara et al found the following during the study:
- A 13% crash reduction in urban areas generally, associated with the introduction of speed cameras.
- A 10.9% reduction in crashes at rural speed camera sites and a 23% reduction in crashes at urban sites.
- A casualty crash reduction of 8.9% during the first week after receipt of tickets from a speed camera operation were found.
- The risk of crash involvement was found to be twice as high for vehicles travelling 10km/h above the mean speed of non-crash involved vehicles and
- nearly six times as high when travelling 20km/h above the mean speed.
- A 12% reduction in crashes for each 1 km/h reduction in mean speed was found. A significant trend of 6% reduction in crashes per year was also found.
- Both mean and 85th percentile speeds have shown a significant decrease associated with the increase in enforcement activity.
Australia (NHMRC Road Accident Research Unit of the University of Adelaide)
The risk of involvement in a casualty crash doubled with each 5 km/h increase in free travelling speed above 60 km/h.
Outcomes at Reduced Travelling Speeds
|
Hypothetical Situation |
% Reduction |
% Reduction |
% Reduction in |
% Reduction in |
|
in no of crashes |
in no. of persons |
average Delta V |
average Crash Energy |
|
|
injured |
||||
|
10km/h speed reduction |
41.5 |
34.6 |
25.5 |
38.7 |
|
5 km/h speed reduction |
15 |
13.1 |
16.1 |
23.6 |
|
Limit 60km/h with total compliance |
28.6 |
30.4 |
11.8 |
21.7 |
|
Limit 50km/h |
32.7 |
26.6 |
24.9 |
37.5 |
Victoria (Newstead, S. & Narayan, S., Monash University Accident Research Centre)
Assessment of the general effects on casualty crash frequency of increasing zoned speed showed a highly statistically significant increase in casualty crash frequency of 9.3% frequency.
For particular speed zone changes,
- the change from 100 to 80 km/h showed a casualty crash reduction of 46%.
- Increased speed zoning from 75 to 80 km/h showed a casualty crash frequency increase of 10.7% and
- the change from 75 to 60 km/h showed a casualty crash frequency reduction of 43%.
South Australia (Kloeden CN, McLean AJ)
- It was also found that 41% of all the casualty crashes investigated would have been avoided if none of the vehicles had been travelling above the average speed
- Total compliance with posted speed limits would have led to a 24% reduction in these casualty crashes.
- Lowering the maximum speed limit on undivided roads to 80 km/h and obtaining the same level of compliance as at present would have reduced the frequency of casualty crashes by 33% overall and by 46% on rural undivided roads with current speed limits above 80 km/h.
|
Hypothetical Situation |
% Reduction |
|
|
in number |
|
|
of Crashes |
|
5 km/h free travelling speed reduction |
30.5 |
|
10 km/h free travelling speed reduction |
46.5 |
|
No speeds above control average |
41.0 |
|
Total compliance with speed limits |
23.8 |
|
80 km/h maximum speed limit on undivided roads with |
33.4 |
|
compliance as at present |
|
|
100 km/h maximum speed limit on all roads with compliance as at |
8.0 |
|
present |
|
|
|
|
United States of America (Michael Paine)
By analysing the data for seat-belt wearing drivers killed in frontal crashes they found that half of all fatalities occurred at an impact speed (or "delta V") of 50km/h or less.
An impact speed of 60km/h is a very severe crash. Higher speeds result in collapse of the passenger compartment and a grave risk of life-threatening injury. Even the most crashworthy vehicles available today are unable to safely protect occupants at frontal impact speeds greater than 70km/h.
For side impact crashes the median impact speed for a fatality is much less than 50km/h because there is less "crumple space" between the intruding object and the occupant.
Crash tests that simulate a vehicle sliding sideways into a pole at 29km/h show that a fatal head injury is extremely likely unless there is an inflatable curtain, or similar device, between the occupant and the pole.
Exceeding the speed limit - whether it be 50km/h or 110km/h - greatly increases the risk of being involved in a serious accident. The risk doubles for each 5km/h over the speed limit.
Recent US research estimates that in at least one third of fatal accident one or more of the vehicles was speeding. In most cases these accidents could have been avoided or made less severe if the vehicles had obeyed the speed limit. This is in agreement with NSW RTA estimates that 40% of accidents are speed-related.
Travelling at 60km/h in a modern car feels safe - but that is an illusion.
An impact at this speed has a very high risk of severe or fatal injuries. This is due to Newton's laws of physics - which, unlike speed limits, cannot be broken - and the frailty of the human body.
NHTSA’s National Center for Statistics& Analysis
Hong Kong, China (S.C.Wong)
The speed limits of a number of sections of roadway were reviewed and increased. Nineteen of them were major roadways.
Their speed limits were raised by 10–20 km/h from 50 to 70 km/h. Before and after studies were carried out to investigate the changes in accident counts.
The accident counts were worse after the increase in speed limits, both for the category of fatal, serious and slight (FSS) accidents, and for the category of fatal and serious (FS) accidents.
Overall, the relaxation of the speed limit from 50 to 70 km/h increased the FSS accident counts by 15% and the FS accident counts by 1%.
The relaxation of the speed limit from 70 to 80 km/h increased the FSS accident counts by 18% and the FS accident counts by 36%.
United States of America (Michael Paine)
Every 5km/h above the speed limit doubles the risk of being involved in a casualty accident. In a 60 zone, travelling at 65km/h doubles the risk and travelling at 70km/h quadruples the risk.
These crash tests suggest that, at impact speeds of around 70km/h, a vehicle reaches physical limits to its ability to protect front seat occupants.
Careful research in South Australia has shown that travelling at 80km/h in a 60 zone increases the chances of being involved in a casualty crash by 32 times, compared with risk when travelling at 60km/h.
40km/h speed zones usually apply where there is an increased risk of hitting a pedestrian. At an impact speed of 60km/h there is almost a 100% chance that a pedestrian impact will be fatal. Also in half of all pedestrian fatalities the motorist has no chance to brake before the impact.
New South Wales
A car is travelling at 40 km/h. Another car is travelling at 50 km/h. Both drivers see a child about 27m ahead, recognize the danger and brake. The car travelling at 40 km/h will stop safely after 26m, avoiding the child. The car travelling at 50 km/h will take an extra 9m to stop, and will still be travelling at 41 km/h when it hits the child.
Even a small difference in vehicle speed can make a large difference to the probability of serious injury.
If a car hits a pedestrian at 50 km/h the car driver is twice as likely to kill the pedestrian than if the car hits a pedestrian at 40 km/h.
For every extra kilometer per hour of speed:
- the stopping distance increases.
- the time to react and avoid a crash decreases.
- the impact of a crash is more severe on the vehicle, driver, passengers and pedestrians.
- the likelihood of serious injury or death increases.
Recommendations:
- The level of enforcement of speed limits in rural areas be increased.
- The tolerance allowed in the enforcement of rural speed limits be reduced or eliminated.
- All currently zoned 110 km/h undivided roads be rezoned to no more than 100 km/h.
- Changing the maximum speed limit to 80 km/h on all two lane rural roads.\
- The level of public awareness of the risk of involvement in a casualty crash associated with speeding be increased with the aim of developing a culture of compliance with speed limits, and support for strict enforcement of speed limits.
Johan Joubert:
Accident Reconstructionist for the RAF
Manager: Accident Analysis Division
TMT Services and Supplies PTY Ltd
Cell: 0826509620
Email: jjoubert@tmtservices.co.za
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