The current study had three phases: the case-control risk estimation phase, the hypothetical crash outcome phase and the examination of the relationship between speed and alcohol. The methods used in each of the phases are presented in the sections below.
This part of the study used a case-control design in which the travelling speeds of vehicles involved in crashes from which at least one person was transported by an ambulance were compared with the speeds of other vehicles passing through the crash locations. The study was conducted in 60 km/h speed limit zones in the Adelaide metropolitan area.
The following criteria were used for the selection of case vehicles:
Cases were restricted to crashes occurring in the Adelaide metropolitan area in 60 km/h speed zones in the interest of uniformity. Higher speed zones would have had fundamentally different speed distributions which would have made the case-control analysis more complicated to perform and the results harder to interpret. Sections of road with advisory speed signs of less than 60 km/h were excluded for similar reasons. Requiring the case vehicle to be a car or car derivative was also specified in an attempt to get a homogeneous sample of cases.
An injury severe enough to require ambulance transport was used as the crash severity criterion. This was based on the importance of injury as opposed to property damage in the crash consequences. It was also a criterion which could readily be used at the scene of the crash.
The case vehicles must have had a free travelling speed prior to the crash. A travelling speed was defined as the speed of a vehicle moving along a mid-block section of road, or with right of way through an intersection, and not slowing to join, or accelerating away from, a traffic stream. This criterion operationally defined travelling speed as it is popularly understood and aimed to ensure that the association between travelling speed and crash involvement was not confused by the inclusion of vehicles executing (necessarily slow) manoeuvres or disobeying right-of-way rules.
Vehicles executing illegal manoeuvres prior to the crash sequence were excluded as cases since they would have had other high risk factors involved. For example, going through a red light increases the chance of a crash independent of travelling speed.
In order to assess the relationship between travelling speed and the risk of crash involvement without possible confounding by alcohol impairment, a case was included only if the driver's BAC was measured and found to be zero.
There also had to be sufficient information available for a computer-aided reconstruction of the crash to be performed. In some cases, vehicles were removed before the arrival of Unit personnel, and/or the point of impact and the final positions of the vehicles could not be determined. These cases were abandoned.
Crashes in which the case vehicle rolled over were excluded due to the greatly increased difficulty in reconstruction. Crashes that occurred while it was raining were excluded due mainly to the difficulty in obtaining control data under the same weather conditions.
In summary, the conditions imposed on the selection of case vehicles were designed to ensure that the study would yield valid estimates of the relative risk of a car travelling at a free speed in a 60 km/h zone becoming involved in a casualty crash compared to the risk for a car travelling at 60 km/h.
The following procedure was used in the investigation of crashes:
The primary method of being advised of the occurrence of a vehicle accident was to listen to the ambulance radio frequency on a hand held scanner. This provided notification of crashes at the same time as an ambulance was informed of the location. However, when an ambulance was despatched from a depot rather than on the road, notification of the ambulance was by phone and not transmitted on the radio. The South Australian Ambulance Service notified the Road Accident Research Unit (RARU) of these crashes by means of a paging service.
Upon the receipt of notification, RARU personnel proceeded directly to the crash scene. On arrival, a quick survey was made to see if the crash met the selection criteria. If it appeared that the case met the criteria, one of the crash investigation team then photographed the scene and the vehicles involved while the other member interviewed police, participants and witnesses. Relevant measurements of the vehicles and the scene were also made. In some cases, vehicles were followed up at crash repairers for further photographs and measurements.
The police accident report on the crash was reviewed to obtain further crash details and verification of zero blood alcohol concentration for drivers who were not breath tested by police or RARU personnel at the scene.
If the crash was found to satisfy the criteria listed in the previous section it was then reconstructed using computer-aided reconstruction techniques to obtain the speeds of the vehicles on impact and, when the necessary additional information was available, the travelling speeds of the vehicles.
All information about the crash, including the crash reconstruction, was then reviewed by an expert panel to determine the suitability of the case and the validity of the reconstruction. Further work on the reconstruction was carried out if necessary with the results being re-reviewed by the panel.
A final decision was then made about the inclusion of the case in the study and the travelling speeds of the vehicles involved.
Data collection was concentrated during the hours of 9:30am-4:30pm, Monday to Friday as these times had the highest number of non-alcohol crashes in Adelaide. Some cases were also collected at nights and on weekends. Peak hour traffic times were avoided due to the reduced incidence of free speed travelling during those times.
The pre-crash travelling speeds of the case vehicles were determined using accident reconstruction techniques. This was made possible by the detailed investigations of the crashes at scene. Features of the crash such as tyre marks, impact points, final positions of vehicles, damage to vehicles, and participant and witness statements were all used in the reconstruction process.
Considerable use was made of the SMAC (Simulation Model of Automobile Collisions) computer program that was developed by Ray McHenry at the (then) Cornell Aeronautical Laboratory of Cornell University about 30 years ago.
SMAC, despite the inclusion of the word "simulation" in the name, is a true reconstruction program in which each step has been developed on the basis of physical testing and studies of vehicle dynamics. In application, it is an iterative program in which a collision between two cars is modelled by starting with the alignment of the cars on impact, which can be determined from the damage to the cars, and estimating impact velocities. The predicted post-impact motions of the vehicles are then compared with the actual motions, deduced from skid marks and the rest positions. If necessary, adjustments are made to the modelled impact geometry and impact velocities until a satisfactory match is obtained.
Having been developed for the National Highway Traffic Safety Administration (NHTSA), the SMAC program was available in the public domain and now is used in a number of commercially available packages, such as ED-SMAC. A simplified pre-processor for SMAC, CRASH (Cornell Reconstruction of Accident Speeds on the Highway) was also developed in the1970s. It enables a less accurate estimate of impact speed to be made from the nature and extent of the collision damage to a car. McHenry, with his son Brian McHenry, has continued to develop both SMAC and CRASH and so the Road Accident Research Unit (RARU) entered into an arrangement whereby the latest versions of both programs were made available and Brian McHenry visited Adelaide for a week to instruct RARU staff in their use. Subsequently, during the study, in some cases the crash data and the RARU reconstruction were sent to Brian McHenry in North Carolina for assessment and, if necessary, a re-run of what they now refer to as the M-SMAC program.
Other methods were also used to establish both impact and travelling speeds. They are all described in some detail in Volume 2 of this report, together with a description of the methods used in each case.
The following criteria were used in the selection of control vehicles:
Control vehicles were matched to the case vehicles based on the location of the crash, direction of travel, the time of day (within 1/2 hour) and the day of week. The control vehicles also had to be car or car derivatives and were required to have a free travelling speed as defined for the case vehicles. Control speeds were also measured in the same weather conditions, which meant a dry road in almost all cases.
Among the controls meeting the selection criteria, a random sample had their speed measured by a member of the Unit using a laser speed meter. This meter can measure the speed of a specified car to within 1 km/h from distances up to 1 km away. The minimum distance from the meter to the car while collecting controls was 200 metres.
The laser speed meter that was used is similar in appearance to a video camera. Even so, every effort was made to avoid alerting the drivers to the presence of speed measuring equipment. Checks were made of the behaviour of drivers travelling in the opposite direction to ensure that none of them were flashing their lights to warn oncoming drivers to slow down.
After the speed of a randomly selected control vehicle had been recorded, two-way radios were used to alert a police random breath testing unit (set up further down the roadway by previous arrangement) of the approach of the desired vehicle. The police then stopped this vehicle and carried out a breath alcohol test on the driver. As the vehicle had been selected randomly, and well before the driver could be seen, this operation was treated as an extension of normal random breath testing operations.
Testing continued until four controls with a driver blood alcohol concentration (BAC) of zero were collected for each case vehicle. These four then formed the control group for that case.
Part way through the study, the use of police to stop the controls became difficult to arrange so only a small proportion of the final control sample had verified zero BAC readings (7.3%). However, it is felt that this does not have a meaningful effect, if any, on the results of the study since only a very few of all the controls tested did in fact have a positive BAC (3%) and the differences in average speed between zero BAC and positive BAC drivers was found to be only a few km/h (see Sections 3.3 and 4.3).
Additional information about the effects of travelling speed was obtained by calculating what the change in velocity at impact (delta V) for those people injured in the crash would have been if the case vehicle had been travelling at a different speed, using methods similar to those applied in the recent Road Accident Research Unit study of speed effects in fatal pedestrian collisions (McLean, et al., 1994).
The calculated travelling speed for the case vehicle was changed, if necessary, according to the hypothetical scenario. All other crash factors were kept constant including points of impacts on vehicles. If the driver of the case vehicle reacted before the collision, by applying brakes, the distance away from the crash site (calculated using reaction times) when he or she reacted was used as the starting point for the scenario. Full details of the calculations used are presented in Volume 2 of this report.
The resulting crash severity, measured by change in velocity at impact, for all persons injured was calculated and compared to the original crash severity.
The following hypothetical scenarios were examined:
Under Scenario 1, all case vehicles were assumed to have a travelling speed of 5 km/h less than their calculated travelling speed.
Under Scenario 2, all case vehicles were assumed to have a travelling speed of 10 km/h less than their calculated travelling speed.
Under Scenario 3, all vehicles that were calculated as travelling over 60 km/h were assumed to be travelling 10 km/h slower; all vehicles calculated as travelling between 50 and 60 km/h were assumed to be travelling at 50 km/h; and all vehicles calculated as travelling under 50 km/h did not have their speeds changed. This scenario was intended as a first approximation estimate of the effect of a change in speed limit from 60 to 50 km/h.
Scenario 4 was the same as Scenario 3 except that the reductions were only applied to crashes occurring on a local street; crashes on main roads did not have their speeds altered. This scenario was intended as a first approximation estimate of the effect of a change in speed limit from 60 to 50 km/h on local streets only.
Under Scenario 5, all case vehicles with a calculated travelling speed above 60 km/h were assumed to be travelling at 60 km/h.
There is virtually no evidence in the literature dealing with the risk of ambulance transport by change in vehicle velocity in a crash so calculating hypothetical reduced rates of injury in these scenarios could not be done. However, under the hypothetical scenarios a large proportion of the crashes would not have happened so this is used as the main measure of the effect of the scenarios.
It was originally planned that the collection of data on the driver's blood alcohol concentration (BAC) and the travelling speed of the control group would also allow the relationship between BAC and travelling speed to be examined. However, the fact that almost all day time drivers were sober made this infeasible in the context of the case-control study.
Therefore a separate study was set up to measure this relationship. The speed of an approaching car was measured using a laser speed meter 200-300 metres before a signalised intersection. When the car stopped at this intersection for a red light, the driver was approached by a Road Accident Research Unit (RARU) staff member and a voluntary breath alcohol test was requested using a procedure developed by RARU (Holubowycz, McLean and McCaul, 1991). Testing was conducted at night in 60 km/h zones at 5 sites in the Adelaide metropolitan area.
3.1 Case-Control Study Risk Estimation
3.1.1 Case Vehicle Selection Criteria
3.1.2 Case Vehicle Investigation Procedure
3.1.3 Determining Speeds of Case Vehicles
3.1.4 Control Vehicle Selection Criteria
3.1.5 Measuring Speeds of Control Vehicles
3.2 Hypothetical Crash Outcome Method
3.3 Relationship Between Speed and Alcohol