4.1.1 Data Collection on Cars Involved in Casualty Crashes

The collection of data on the cars involved in casualty crashes (the cases) began on 3 January 1995 and continued through to 20 December 1996 on a 5 day per week on-call basis. A small number of cases was also added to the study in early 1997.

Table 4.1 lists the crash notifications that the Unit received and responded to during the period of the study together with reasons for the exclusion of crashes. In a further 63 cases there was no evidence remaining from the crash on arrival at the scene.

While most of the excluded crashes did not fit the selection criteria, a number of cases were excluded because of insufficient information for crash reconstruction. Most of these crashes were excluded by the investigators at the scene of the crash and no detailed record was kept of the specific reasons for exclusion. However, the following reasons were typical of why these cases were excluded:

• There was not enough information at the site to successfully reconstruct the collision (vehicle impact positions or final positions may have been uncertain or vehicles may have been moved from the site).
• Rear end collisions where the vehicle that was struck may have been in motion.
• Conflicting information supplied by drivers, witnesses and or police.
• Hit and run pedestrian/bicycle/motorcycle accidents.
• Vehicle occupants absconded after the accident (eg stolen vehicle or DUI).
• Pedestrian or cyclist accidents where essential information was lacking (such as the point of impact and the final rest position of the cyclist/pedestrian/car).
• A few cases involved complicated events that could not be successfully analysed by SMAC.

4.1.2 Data Collection on Non-Crash Involved Cars

Figure 4.1 shows the speed distribution of the vehicles involved in casualty crashes (cases) and Figure 4.2 shows the corresponding information for non-crash involved vehicles (controls).

Cars involved in casualty crashes (cases) were generally travelling faster than cars that were not involved in a crash (controls): 68 per cent of crash involved cars were exceeding 60 km/h compared to 42 per cent of those not involved in a crash (Table 4.2). The difference was even greater at higher speeds: 14 per cent of crash involved cars were travelling faster than 80 km/h in a 60 km/h speed zone compared to less than 1 per cent of those not involved in a crash. The crash-involved cars were almost 10 times more likely to have been travelling faster than 70 km/h than were the non-crash-involved cars (29% vs 3%).

4.1.4 Travelling Speed and the Relative Risk of Involvement in a Casualty Crash

The risk of being involved in a casualty crash is very low. In South Australia in 1994, there were 556 casualties per 100,000 population, and 88 casualties per 10,000 vehicles during that year (Office of Road Safety, 1996). However, even though the average risk may be low, proportional differences in that risk between, say, drivers travelling at 80 km/h and those travelling at 60 in a 60 km/h speed limit zone may be very large.

In this section we present the risk of involvement in a casualty crash at specified speeds relative to the risk for drivers travelling at 60 km/h. The speeds of the cases (the crash-involved drivers) and the controls (those not involved in a crash) are grouped in 5 km/h intervals as shown in Table 4.3.

The relative risk is calculated as in the following example taken from Table 4.3:

The relative risk (R.R.) of involvement in a casualty crash at a travelling speed of 70 km/h compared to 60 km/h is calculated as follows:

That is, a driver travelling at 70 km/h in a 60 km/h speed zone has a risk of being involved in a casualty crash that is more than four times greater than that of a driver travelling at the speed limit.

This method of calculating the relative risk makes use of the fact that crash involvement is a rare event, as noted above. The figure of 4.16 is actually the relative odds of involvement in a casualty crash. However, the relative odds are virtually the same as the relative risk when dealing with rare events (MacMahon and Pugh, 1970).

As in any estimate of this type, it is not certain that the estimate of relative risk obtained is an accurate representation of the 'real' relative risk. However, it is possible to calculate the range of values that probably includes the 'real' relative risk (Gart, 1962) and the limits of this range are shown in Table 4.3. For the above example, the 95% confidence limits are 2.12 and 8.17. This means that the 'real' relative risk has a 95% probability of being within the range from 2.12 to 8.17. If, as here, the interval between the confidence limits does not include 1.00 then it can be said that the risk of involvement in a casualty crash at the specified travelling speed (here it is 70 km/h) is statistically significantly different from the risk at a travelling speed of 60 km/h.

A statistically significant difference is not necessarily large enough to be of practical importance. The results listed in Table 4.3, however, show that even a travelling speed of 65 km/h doubles the risk of involvement in a casualty crash. An increase in risk of that magnitude is clearly of practical importance.

None of the travelling speeds below 60 km/h was shown to be associated with a risk of involvement in a casualty crash that was statistically significantly different from the risk at 60 km/h. There was some indication that the risk decreased somewhat to 50 km/h and then increased to greater than one at 40 km/h but the confidence intervals show that this trend could well have arisen purely from random variation.

Above 60 km/h, however, there is a steady increase in risk of involvement in a casualty crash with increasing travelling speed such that the risk approximately doubles with each 5 km/h increase in travelling speed.

The information in Table 4.3 is presented graphically in Figure 4.3. The representation of the speed data in 5 km/h intervals provides a clear picture of the change in risk by speed. The smoothness of the resulting curve, the closeness of the fit with an exponential curve (R2 = 0.993 for points at 60 km/h and above), and the fact that the rate of increase in risk is unchanged with wider class intervals, are consistent with the curve in Figure 4.3 being a reasonable representation of the real association.

4.1.5 Free Travelling Speed Crash Types

Each crash involving a free travelling speed case vehicle was classified into one of 11 crash types. In crashes with multiple case vehicles, the crash type was classified separately for each vehicle. The average travelling speed of the case vehicles and the associated controls in each category was also calculated. The results are shown in Table 4.4.

The most common crash types in the sample were an oncoming vehicle turning right across the path of the free travelling speed vehicle (36%) and a vehicle turning right from the side street on the left of the free travelling speed vehicle (15%). These two categories accounted for over half of all the crash types.

In each of the 148 crashes in this study, at least one person was injured sufficiently to require transport to a hospital. In total, 237 persons received an injury from these crashes.

Table 4.5 shows the outcome, mostly in terms of the level of treatment, of the injured persons in the crashes investigated (based on police report information). Those cases listed under "Transported by ambulance" were known to have been transported to hospital but it was not listed on the police report whether they were treated in the casualty department and discharged or admitted to the hospital for longer term treatment.

4.2.2 Location of Crashes

Table 4.6 shows the distribution of the crashes, and the number of persons injured, by the type of road on which the free speed vehicle was travelling when the crash occurred. In this sample of free travelling speed crashes, only 12.7 per cent of the persons injured were involved in casualty crashes on local streets (defined as all roads other than those designated as main traffic routes or alternate traffic routes in the Adelaide UBD Street Directory).

4.2.3 Hypothetical Outcomes at Reduced Travelling Speeds

For each crash, five hypothetical speed reduction scenarios were applied to the free travelling speed of the case vehicle (or to multiple case vehicles if appropriate). The results are expressed in terms of four factors: an estimated reduction in the number of crashes and persons injured due solely to those crashes not happening; and, in those crashes that would still have occurred, the reduction in the change in velocity (delta V) and the crash energy experienced by the injured parties (see Table 4.7).

Note that the percentage reduction in the number of persons injured is an underestimate since most of the crashes that would still occur under a hypothetical lower travelling speed situation would have occurred at a lower speed than was actually the case and would therefore have had a lower chance of causing injury. For example, some of the cases under the hypothetical scenarios would have had an impact speed of only a few km/h so, even though the crash would still have taken place, it is almost certain that no injury would have resulted. Also, some of the drivers who lost control of the case vehicle would probably not have done so under a hypothetical lower travelling speed situation and in some cases the other vehicle may not have misjudged the case vehicle's speed and created a crash situation.

A uniform 10 km/h reduction in the travelling speeds of the case vehicles offered the greatest reduction in the number of crashes (42%) and persons injured (35%) and also offered the greatest reduction in crash energy experienced by injured parties in crashes that would still have taken place (39%). The 5 km/h reduction scenario had much less effect on the elimination of crashes (15%) but still reduced the average crash energy level experienced by the injured parties in those crashes that still would have occurred by 24 per cent.

The current speed limit of 60 km/h, enforced to ensure total compliance, and a hypothetical speed limit of 50 km/h with the present level of compliance, also showed large reductions in the number of crashes and persons injured, and the average delta V and crash energy levels experienced by the injured parties in crashes that would still have happened.

An alternative estimate of the effect of the elimination of speeding (limit 60 km/h with total compliance) can be derived from the data in Table 4.3. The Table shows that 93 (62%) of the case vehicles were speeding (speed greater than the 58-62 km/h band). If none of the case vehicles had been speeding (ie. their relative risk was reduced to 1.0), fewer casualty crashes would have occurred. Working back from the relative risk figures, we would expect that 50 per cent of the crashes in the 65 km/h band might have been avoided (or been reduced from a casualty crash to one not requiring ambulance transport), rising to 98 per cent of the 85 km/h crashes, and virtually all of the crashes involving vehicles above 87 km/h.

By applying this method to all of the cases exceeding 62 km/h (Table 4.8) it can be seen that the elimination of speeding would be expected to reduce free travelling speed casualty crashes by about 46 per cent. This is consistent with the equivalent percentage calculated in Section 4.2.3 (29%) being a considerable underestimate due to that method only taking into account crashes avoided, and not a reduction in the severity of those crashes that would still occur. Also any effects of driver loss of control due to speeding and other drivers failing to realise how fast the case car was travelling are allowed for in Table 4.8 but not in the hypothetical scenarios presented in Section 4.2.3.