Travelling Speed and the Risk of Crash Involvement

RESULTS

The results of the three phases of the study are presented below. The case-control study results show the relationship between travelling speed and the risk of involvement in a casualty crash. The hypothetical crash outcome results show the likely effect of reductions in travelling speed and the results of the third phase of the study show the relationship between travelling speed and a driver's blood alcohol concentration.

4.1 Travelling Speed and the Risk of Involvement in a Casualty Crash

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.

Table 4.1
Crashes Attended and Reasons for Exclusion from the Study

Crashes AttendedNumber of Crashes
Total number of crashes attended952
Crashes excluded804
- No ambulance transport required325
- Case vehicle was not a car or car derivative148
- Case vehicle did not have a free travelling speed148
- Case vehicle doing illegal manoeuvre26
- Crash due to medical condition of driver23
- Site not in a 60 km/h zone18
- Not a vehicle accident8
- Case driver had a positive blood alcohol concentration5
- Case vehicle rolled over4
- Insufficient information for crash reconstruction99
Valid crashes148
Note: 3 crashes yielded 2 case vehicles each giving a total of 151 total cases

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:

4.1.2 Data Collection on Non-Crash Involved Cars

The collection of data on the non-crash involved cars (the controls) was normally carried out a week or two after the crash had occurred. In rare cases, there was a delay of up to a few months due to uncertainties in the case, bad weather or very few potential control cars passing through the site of the crash.

4.1.3 Comparing the Travelling Speeds of Cases and Controls

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).

Figure 4.1
Travelling Speed Distribution of Casualty-Crash-Involved Vehicles (Cases)

Figure 4.1

Figure 4.2
Travelling Speed Distribution of Non-Crash-Involved Vehicles (Controls)

Figure 4.2

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%).

Table 4.2
Percentage of Vehicles
Travelling Above the Given Speeds
Speed (km/h)Per cent above speed
CasesControls
5094.788.7
6067.542.1
6547.712.9
7029.13.0
7519.20.7
8013.90.5
858.60.0
906.00.0
953.30.0
1002.60.0

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.

Table 4.3
Travelling Speed and the Risk of Involvement in a Casualty Crash
Relative to Travelling at 60 km/h in a 60 km/h Speed Limit Zone

Nominal
Speed
Speed
Range
No. of
Cases
No. of
Controls
Relative
Risk
Lower
Limit*
Upper
Limit*
3533-37040--
4038-42151.410.1612.53
4543-474300.940.312.87
5048-525570.620.231.67
5553-57191331.010.541.87
6058-62292051.001.001.00
6563-67361272.001.173.43
7068-7220344.162.128.17
7573-779610.603.5231.98
8078-829231.816.55154.56
8583-878156.556.82468.77
-88+110infinite--
Total151604
* 95% confidence limits of the estimated relative risk

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

Travelling Speed
60 km/h70 km/h
Cases2920
Controls20534

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:

Equation

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.

Figure 4.3
Travelling Speed and the Risk of Involvement in a Casualty Crash
Relative to Travelling at 60 km/h in a 60 km/h Speed Limit Zone

Figure 4.3

Note: Relative risk at 60 km/h set at 1.00.
95 per cent confidence intervals are shown by the vertical lines.

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.

Table 4.4
Crash Type and Average Travelling Speed
Crash TypeNumber
of Cases
Per cent
of Cases
Average Case
Speed (km/h)
Average Control
Speed (km/h)
Oncoming vehicle turned right across path5536.468.959.0
Vehicle entering from left turned right across path2315.263.058.6
Loss of control followed by collision149.382.663.3
Rear end collision with vehicle in front149.363.560.4
Hit pedestrian or bicyclist127.962.861.6
Vehicle crossing in front from right to left96.065.256.4
Vehicle doing U-turn in front85.365.160.6
Vehicle crossing in front from left to right74.662.760.3
Hit by an out of control vehicle74.666.465.0
Vehicle on right turned right into path10.766.061.3
Side swiped vehicle travelling in the same direction10.792.058.0
Total151100.067.659.9

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.

Disregarding the last two categories in Table 4.4 because of the single cases, the crash types associated with the highest free travelling speeds were: losing control of the vehicle followed by a collision (average speed = 83 km/h); and having an oncoming vehicle turn right across the path of the free travelling speed vehicle (average speed = 69 km/h).

4.2 Hypothetical Crash Outcomes at Reduced Travelling Speed

Additional information about the effects of travelling speed was obtained by calculating what the hypothetical outcome for the vehicles and those people injured in the case crashes would have been if the case vehicle had been travelling at a different speed.

4.2.1 Injuries Sustained in the Crashes

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.

Table 4.5
Injury Outcome
Injury OutcomeNumberPer cent
Injured but not treated31.3
Treated by private doctor83.4
Transported by ambulance2510.5
Treated at hospital13356.1
Admitted to hospital6226.2
Fatality62.5
Total237100.0

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).

Table 4.6
Type of Road by Number of Crashes and Persons Injured
Road TypeCrashesPersons Injured
NumberPer centNumberPer cent
Main road12785.820787.3
Local street2114.23012.7
Total148100.0237100.0

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).

Table 4.7
Hypothetical Outcomes at Reduced Travelling Speeds

Hypothetical Situation% Reduction
in number
of Crashes
% Reduction
in number
of Persons
Injured*
% Reduction
in average
Delta V**
% Reduction
in average
Crash
Energy**
10 km/h speed reduction41.534.625.538.7
5 km/h speed reduction15.013.116.123.6
Limit 60 km/h with total compliance28.630.411.821.7
Limit 50 km/h with compliance as at present32.726.624.937.5
Limit 50 km/h on local streets only with compliance as at present6.14.22.84.7
* Reductions due solely to the crash not happening under the scenario.
** Average reduction for persons injured in crashes that would still have happened under the scenario.

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.

A hypothetical speed limit of 50 km/h in local streets, while having a significant effect on local street crashes, had only a small effect on the set of crashes as a whole due mainly to the very small proportion of these crashes and the resulting injuries which occurred on local streets (see Table 4.6).

4.2.4 Estimated Effect of Eliminating Speeding Vehicles Based on Risk Estimates

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.

Table 4.8
The Effect of the Elimination of Speeding
on Free Travelling Speed Casualty Crashes

Nominal
Speed
Speed
Range
No. of
Cases
Relative
Risk
Expected
Cases*
% Reduction
in Crashes
3533-370000
4038-4211.4110
4543-4740.9440
5048-5250.6250
5553-57191.01190
6058-62291.00290
6563-67362.0018.050.0
7068-72204.164.876.0
7573-77910.600.890.6
8078-82931.810.396.9
8583-87856.550.198.2
-88+11infinite0.0100.0
Total15182.045.6
* Assuming all relative risks for speeds above 62 km/h are reduced to 1.00

4.3 Relationship Between Speed and Alcohol

A total of 1083 drivers were approached for a breath sample at a red traffic signal after their mid-block approach speed had been measured. Of these, 1002 provided a sample (7.5% refusal rate) and 169 (16.9%) were found to have been drinking. Table 4.9 summarises the speeds associated with specified blood alcohol concentration (BAC) groups and Figure 4.4 shows the full speed distributions for sober drivers compared to BAC positive drivers. It appears from these results that higher BAC levels are associated with slightly higher travelling speeds although the average difference in speed is only a few kilometres per hour.

Table 4.9
Speed Distribution by Driver's Blood Alcohol Concentration (BAC)

BAC GroupNumber
of Cases
Average
Speed (km/h)
% slow
< 55 km/h
% normal
55 - 65 km/h
% fast
> 65 km/h
zero83360.0412.773.913.3
.0057860.6417.964.117.9
.010 - .0457161.1414.167.618.3
.050+2062.90*-75.025.0
Total positive16961.12*14.266.918.9
* Statistically significantly different from average speed of sober drivers (p <= 0.05)
Note: BAC was measured in increments of .005 g/100mL

Figure 4.4
Cumulative Speed Distributions for Zero BAC and Positive BAC Drivers

Figure 4.4


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