Peter Christensen’s research while affiliated with Transportøkonomisk institutt, TØI and other places

Most road safety studies rely on summary measures of exposure. The term summary measure denotes any aggregate indicator of exposure that does not directly identify and count the number of opportunities for accidents to occur. This paper shows how elementary units of exposure can be developed on the basis of known aggregate measures, such as AADT. An elementary unit of exposure refers to any event that generates an opportunity for an accident to occur. Four such events are identified: (1) Encounters, i.e. vehicles passing each other in opposite directions of travel; (2) Simultaneous arrivals at points of intersection between potentially conflicting directions of travel, in particular vehicles entering intersections at the same time or within a very short time interval; (3) Change of travel lane on multi-lane highways; (4) Braking or stopping. These events describe traffic movements prior to a wide range of crash types. The only major group of accidents that is not directly related to particular events is running off the road. The number of events expected to occur for each of the four types identified is estimated by relying on the assumptions: (A) that AADT is known − when estimating the number of events, mean hourly volume (AADT/24) is used; (B) that vehicles or road users arrive at a point of potential conflict according to a Poisson process, and (C) that simultaneous arrivals within a very short time interval (such as 1 second) have the potential for generating a conflict. It is found that the number of encounters and simultaneous arrivals in intersections increases considerably faster than AADT. The number of events that may generate conflicts involving lane changes or braking or stopping increases more slowly than AADT.

Many motorized countries use fixed penalties to deter the most common traffic violations. Fixed penalties are usually given at the spot by a police officer. If the offender accepts the fixed penalty, no court hearing or trial is held. During the years 1995-2004, the rates for fixed penalties for traffic offences in Norway increased substantially. This paper evaluates the effects on compliance of these increases. Regression analysis was performed to determine the effects of increases in fixed penalties. For speeding in general, no effect of increasing fixed penalties can be found. For speeding close to speed camera sites, there is a weak tendency for the violation rate to go down. This tendency is not statistically significant at conventional levels. For seat belt wearing, wearing rates are found to increase as fixed penalties have increased. In recent years, however, enforcement of the seat belt law has stepped up, making it impossible to separate the effect of enforcement from that of fixed penalties. It has been suggested that the police may adapt to stricter penalties by reducing enforcement or by adopting larger tolerance margins for violations. Available evidence does not support this hypothesis.

An extensive programme of periodic motor vehicle inspection was introduced in Norway after 1995, when the treaty between Norway and the European Union (EU) granting Norway (not a member of the EU) access to the EU inner market took effect (The EEA treaty). This paper evaluates the effects on accidents of periodic inspections of cars. Trucks and buses were not included in the study. Negative binomial regression models were fitted to data on accidents and inspections created by merging data files provided by a major insurance company and by the Public Roads Administration. Technical defects prior to inspection were associated with an increased accident rate. Inspections were found to strongly reduce the number of technical defects in cars. Despite this, no effect of inspections on accident rate were found. This finding is inconsistent with the fact that technical defects appear to increase the accident rate; one would expect the repair of such defects to reduce the accident rate. Potential explanations of the findings in terms of behavioural adaptation among car owners are discussed. It is suggested that car owners adapt driving behaviour to the technical condition of the car and that the effect attributed to technical defects before inspection may in part be the result of a tendency for owners who are less concerned about safety to neglect the technical condition of their cars. These car owners might have had a higher accident rate than other car owners irrespective of the technical condition of the car.