The study presented here has the advantage of studying the same locations at different hourly traffic flows and having data on hourly traffic flow separated on the number of cars and lorries. The aim of the study is to estimate the relationship between the accident frequency and the traffic flow, treating the traffic flow in two different ways. In the first model the traffic flow is defined as the number of vehicles per hour treating the traffic flow as if consisting of homogenous vehicles. In the second model the traffic flow is defined as the number of cars and the number of lorries per hour in order to take into consideration that different road user groups may affect the traffic safety differently. Data is collected from 83 road sections in rural areas of Sweden, where the Swedish National Road Administration continuously counts the number of passing vehicles. The assumption is made that the counted traffic flow at a stationary place is valid along the section. Information on police reported accidents with personal casualties, which have occurred on the studied road sections, is collected from 1989 to the middle of 1995. Given the time and date of the accidents that have occurred on the road sections, the hourly traffic flow that prevailed at the time of each accident is obtained. Only accidents occurring on sections without intersections are included. The relationship between the accident frequency and the traffic flow is estimated empirically using regression analysis. The result generally suggests a good fit for both the Poisson and the Negative Binominal regression models. The result of this study indicates that there is important information lost if no consideration is taken of differences between vehicle types when estimating the marginal effect of the traffic flow. The accident rate decreases when the traffic flow is treated as if homogeneous. Since cars constitute the main part of the traffic flow, one may expect the same result when studying the flow of cars. However, when cars are studied the results suggest that the accident rate is constant or increases. The result with respect to the flow of lorries may be an effect of people's unease when sharing the road-space with a lorry. The presence of a lorry may cause other road users' attention to increase, which in its turn lowers the number of accidents occurring. For the covering abstract see ITRD E126595.
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