Involvement in a crash is often a consequence of a large number of inter-related causal factors that cannot all be quantified after the event. This study was not designed to investigate the mechanisms by which increases in travelling speed lead to increases in crash risk but a number of possible mechanisms are apparent.

Travelling Speed, Reaction Distance and Braking Distance

In most of the crashes included in this study the driver of the case vehicle attempted, clearly unsuccessfully, to avoid the crash by braking. One hundred and seven, or 71 per cent, of the 151 case vehicles skidded under emergency braking before the crash. Two of the 151 vehicles were equipped with antilock brakes but their drivers were among the 44 who were thought not to have braked, either because they did not have time to attempt any avoiding action or because they were not aware that a crash was imminent.

We refer here to "reaction distance" rather than the more common term "reaction time" because we find that it emphasises the reasons why small differences in travelling speed can result in very large differences in impact speed. We have discussed this phenomenon at some length in a report on a previous study of the role of vehicle travelling speed in fatal pedestrian accidents (McLean et al, 1994).

In calculating the reaction distance we have assumed that the time taken by the driver to identify that a crash is likely, decide on avoiding action, and implement that action, is 1.5 seconds (the reasons for selecting 1.5 seconds are presented in McLean et al, 1994). In some cases it may have been less, in others more. (There were some cases in which the driver had no warning of the crash before the impact and hence no opportunity to react.) The reaction distance is therefore assumed to be that distance travelled by the vehicle in 1.5 seconds and so it is directly proportional to the travelling speed of the vehicle. At 60 km/h it is 25 metres; at 70 km/h, 29 metres.

Given that the case vehicles were chosen because they had a free travelling speed, among other criteria, their speed is essentially unchanged over the reaction distance. After 25 metres at 60 km/h it is still travelling at 60 km/h. After 29 metres at 70 km/h it is still travelling at 70 km/h. These differences do not appear to be particularly worthy of comment. However their importance becomes apparent when the second stage of the precrash sequence, emergency braking, is considered.

Although it is not uncommon for drivers to claim that they swerved to attempt to avoid a collision, in practice any attempt at avoiding action almost invariably involves emergency braking. In the absence of antilock brakes, braking in an emergency from the speeds seen in this study results in the wheels locking and the vehicle skidding, even for highly skilled drivers. (An emergency situation in normal traffic is, by definition, unanticipated, unlike some apparently similar situations on the race track.) Skidding under emergency braking is accompanied by the loss of steering control and hence the loss of the ability to steer away from an object in the path of the vehicle.

Braking distance, from travelling speed to a standstill, is proportional to the square of the speed. From 60 km/h (under the assumptions set out in Volume 2 of this report) the braking distance is 24 metres, from 70 km/h it is 31 metres.

When the reaction distance is added to the braking distance, it can be seen that from 60 km/h it requires 49 metres to stop in an emergency, whereas from 70 km/h 60 metres are needed (a 22% increase). Obviously, a driver travelling at 60 km/h will be involved in fewer crashes (avoiding those in the zone from 49 to 60 metres) than one travelling at 70 km/h.

However, even this comparison understates the importance of a 10 km/h difference in travelling speed. For example, consider two cars that are travelling side by side at a given instant, one travelling at 60 km/h and the other overtaking at 70 km/h. Suppose that a child runs onto the road at a point just beyond that at which the car travelling at 60 km/h can stop. The other car will still be travelling at 45 km/h at that point. A difference in travelling speed of 10 km/h can mean a difference in impact speed of 45 km/h, or no impact and one at 45 km/h.

In this study we recorded far greater differences in speed between some of the cases and the control cars than in the previous example and this is reflected in even more extreme differences in speed at the moment when the slower car has stopped under emergency braking. For example, if a car travelling at 100 km/h is overtaking a car travelling at 60 km/h at the moment when both drivers realise that they have to try to stop, when the slower car has stopped the faster car will still be travelling at 94 km/h.

The calculations earlier in this section assume that the relationship between stopping distance and crash risk is linear. If other road users appeared more or less at random in a vehicle's path then, other things being equal, a 22 per cent increase in stopping distance (from 60 to 70 km/h) would mean a 22 per cent increase in collisions. However, it seems reasonable to assume that other road users do not always behave at random, and that they are more likely to appear in a vehicle's path at a range of 50-60m than at 0-10m or 10-20m. If this is the case, then a 22 per cent increase in stopping distance may increase crash risk by much more than 22 per cent.

Impact Speed and Crash Energy

The energy of a vehicle that must be dissipated in a crash is proportional to the square of the speed of the vehicle at impact. This means that small differences in impact speed are associated with large differences in crash energy, and correspondingly large differences in injury potential.

In a situation where no speed reduction action is taken (29 per cent of cases in this study) the impact speed is the same as the travelling speed so the energy of the impact is the square of the travelling speed and we would expect the risk of an injury crash to increase rapidly with travelling speed.

Travelling Speed and Loss of Control

A number of cars involved in crashes in this study were travelling at very high speeds (greater than 90 km/h) when the driver lost control of the vehicle. This coupled with the total absence of any of the cars not involved in crashes travelling at these speeds, indicates that very high speeds are associated with extremely high risks of losing control of the vehicle and subsequent crashes and injuries.

Travelling Speed and Driver Expectancies

It is likely that a driver of a vehicle that is travelling unusually fast may create dangerous situations. This can happen when another driver assumes that the approaching speeding car is travelling at about the same speed as other traffic on that road.

Some evidence that such a phenomenon exists was provided by the in-depth study of a representative sample of accidents conducted in Adelaide 20 years ago (McLean, Offler and Sandow, 1979). In 35 collisions at Stop sign controlled intersections it was concluded that only two resulted from a driver failing to observe the Stop sign. In the remaining cases a driver stopped at the sign and then moved off into the path of an approaching car on the through road. The drivers involved in all of the crashes in that study were routinely asked about any prior convictions for speeding in the previous five years. The results are shown in Table 5.1, where it can be seen that the drivers on the through roads were four times more likely to have reported a prior conviction for speeding than the drivers who were, in almost every case, moving off from a Stop sign.

A Combination of Factors Relate Travelling Speed to Crash Risk

The factors discussed here often have a cumulative, and probably a synergistic effect on the risk of involvement in a casualty crash. For example, a speeding vehicle is likely to have its speed misjudged by another driver, thereby creating a crash situation, in which the speeding vehicle will travel further during the reaction time of its driver, will lose less speed under emergency braking, and will crash at a comparatively greater speed with much greater crash energy.

The results presented in Section 4.1 are our best estimates of the relationship between travelling speed and the risk of involvement in a casualty crash. We are aware of a number of matters which could have affected the validity of the risk estimates and they are discussed here.

Crash Severity

Higher travelling speeds are almost certainly related to more serious injuries in the resulting crashes. Insofar as we have presented our risk estimates in terms of free travelling speed casualty crashes as a whole, any bias towards more severe crashes could introduce a corresponding bias towards higher risk estimates. In fact it may be more precise to say that such an effect would mean that our risk estimates are based on a slightly higher than average level of crash severity.

In the study reported here we have attempted to obtain a reasonably representative sample of crashes to which an ambulance was called and which resulted in at least one person being transported to hospital, as well as one of the vehicles being a passenger car which had a free travelling speed. The Adelaide in-depth accident study is an example of the type of study design needed to be confident of obtaining a representative sample of crashes (McLean and Robinson, 1979). That approach is costly in terms of on-call time and so the approach adopted for this study relied on the fact that we usually did not know the type or severity of the accident we were responding to when we were notified of its occurrence by the ambulance service.

Excluded Cases

A number of cases that met the selection criteria of the study had to be excluded for various reasons and where this exclusion was potentially related to travelling speed it may have been a source of bias or a caveat on the level of crash severity as mentioned above.

The more serious crashes were more likely to have the necessary information available for reconstruction of the crash. Because the police tended to keep serious crash scenes intact for longer and to mark out positions of vehicles and physical evidence in more detail, while minor crash scenes were cleaned up quickly and the vehicles taken away, we were more likely to arrive at the scene of a serious crash in time to be able to collect the evidence needed for input to the crash reconstruction program. So while ambulance transport from the crash scene was the nominal severity level of the crashes, it is likely that this set of cases deals with slightly more serious crash outcomes in terms of the severity of the injuries and the damage to the vehicles. There are other matters which may have a bearing on the validity of the sample of cases selected for use in this study. For example, the selection of case vehicles on the basis of free travelling speed obviously placed considerable demands on the judgement of the investigators at the crash scene. In general, if there was some uncertainty the crash was still investigated and, if it was considered necessary, rejected at a later date when all of the available evidence relating to whether or not the case vehicle had a free travelling speed before the crash could be considered.

Case Vehicle Speed Calculation

The validity of the risk estimates depends on the accuracy of the reconstruction of the travelling speed of the case vehicles. While non-systematic errors will just increase the variability of the risk estimates, systematic errors have the potential to bias the risk estimates.

The crash reconstruction method used in this study depends primarily on the physical evidence left at the scene after the crash event. The greatest potential for bias in this respect is due to the inability of the method to take into account speed lost before impact due to braking without leaving skid marks. It is possible that some proportion of the 29 per cent of the case vehicles included in the study that showed no physical evidence of braking before impact actually did brake without leaving skid marks. This would mean that their travelling speeds would have been underestimated leading to a bias in the overall risk estimate.

There were also a few cases where the damage to the vehicles indicated that the case vehicle was braking at impact (eg: lower than usual front bumper height) but there was no physical evidence of braking at the scene. In this situation, the case was rejected because while it was known that speed was lost before impact, this speed could not be quantified. If these cases differed systematically in their travelling speed from the other cases, their exclusion could have biased the risk estimates. However, there is no reason to believe that such a bias exists in these cases.

It is emphasised that the travelling speed listed for each case is our best estimate of the actual speed. We believe that we have made use of the best available methods of crash reconstruction, both computer-aided and in interpretation of the physical evidence at the crash scene and the damage to the vehicles involved. Nevertheless we recognise that the final decision on the travelling speed of a case vehicle is a matter of judgement that may have involved some unknown bias on the part of the investigators. In Volume 2 we present the evidence that we used in arriving at our estimate of travelling speed in each case.

It is sometimes claimed that the only truly accurate way to estimate vehicle travelling speed before a crash is by the use of a crash recorder installed in the vehicle. Such devices are available which are designed to retain a record of the speed of the vehicle for a specified time interval before a crash. It is obvious that such a device should provide a very accurate record of the travelling speed of the vehicle, but it cannot provide information on whether or not that speed was a free travelling speed.

What appears to be less obvious is that a valid estimate of the relationship between travelling speed and the risk of crash involvement requires control data on a sample of similar vehicles. If the crash recorders are fitted to a fleet of cars owned by a public authority, for example, the relevant control speeds would be those of other vehicles in that fleet at the crash location at the same time of day and day of week (and lighting and weather conditions).

Matching of Cases and Controls

Although the controls were matched to the cases based on location, time of day and day of week, there may have been other factors that affected the travelling speed of the cases that did not affect the controls. The most obvious of these would be the presence of another vehicle turning in front of the cases, and not in front of the controls. The presence of this vehicle may have caused the case vehicle to either slow down because of a perceived risk or may have caused them to speed up in an effort to claim that space on the road. The existence and direction of any such bias is not known.

Risk Factors Other than Travelling Speed

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. 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.

Our results show that the risk of involvement in a casualty crash is twice as great at 65 km/h as it is at 60 km/h, and four times as great at 70 km/h. Increases in risk of such magnitude would appear to be sufficient 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.

Although the risk increases rapidly with increasing speed, the 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 4.3. In this study, more than two thirds of the crashes involving speeding cars occurred at a speed that was below 75 km/h, because more drivers are travelling in the speed range from 61 to 74 km/h than above the latter speed.

A large proportion of the injuries sustained in crashes in this study would have been avoided had the case vehicles been travelling at a slower speed. The change in velocity in the crash (delta V) and the severity of the injuries that would still have occurred would have been markedly reduced. Even modest reductions in travelling speeds can have a profound beneficial effect on crash and injury frequency.

Despite the considerable magnitude of the predicted benefits, shown in Table 4.7, it is probable that they are still considerable underestimates. This is because we have only considered the effect of reduced travelling speed on the elimination of crashes in the collision configuration that we actually observed. Many of the hypothetical crashes would not have happened because the driver would not have lost control at a lower speed, the other vehicle may not have misjudged the case vehicle's speed and created a crash situation, and the lowered impact speeds of the crashes that did still happen would have produced fewer and less severe injuries.

Table 5.2 shows the relative risks of involvement in a casualty crash by travelling speed from this study (Table 4.3) and by blood alcohol concentration from the study referred to above. This comparison is unique. Case control studies on speed and alcohol have not been conducted in the same city anywhere else in the world.

Tables 5.3 and 5.4 show the relevant sections of the South Australian Road Traffic Act 1961, as at 3 February 1997, that deal with the penalties for drink driving and speeding.

Given that the relative risk of involvement in a casualty crash at 72 km/h is similar to that for a blood alcohol concentration (BAC) of 0.12 it is more than a little incongruous that the penalty for the BAC offence is a $500-$900 fine and automatic licence disqualification for at least six months while the penalty for the speeding offence is only a $110 fine.

It can be argued that driving at an illegal speed in an urban area is rarely applicable to a whole trip, unlike driving with an illegal blood alcohol concentration. This is not necessarily as great a difference as might be assumed when only those parts of a trip where the vehicle is in motion are considered. Furthermore, the frequency of driving at an illegal speed is very much greater than that of driving with an illegal blood alcohol concentration.

Given that the risks associated with speeding and illegal drink driving are similar, and speeding is more common, why isn't speed listed as a cause of accidents more often than drink driving? The answer probably lies in the fact that it is a comparatively straightforward matter for a police officer to measure a driver's blood alcohol concentration after an accident whereas the estimation of the speed of a vehicle before the crash is rarely a straightforward matter, as indicated in Volume 2 of this report. One consequence of this underestimation of the role of speed in accident causation is the marked disparity in the penalties associated with speeding and illegal drink driving, based on the risk of involvement in a casualty crash.