【正文】 application is not possible. Accident blackspot analysis is often hindered by this limitation, ., if such a test is applied to find out whether the number of accidents at a particular location is higher than average. The procedure described can also be used if the accidents are classified according to a number of characteristics to find promising safety targets. Not only with aggregation, but also with disaggregation the Poisson assumption holds, and the accident numbers can be tested against each other on the basis of the Poisson assumptions. Such a test is rather cumbersome, because for each particular case, . for each different Poisson parameter, the probabilities for all possible outes must be puted to apply the test. In practice, this is not necessary when the numbers are large. Then the Poisson distribution can be approximated by a Normal distribution, with mean and variance equal to the Poisson parameter. Once the mean value and the variance of a Normal distribution are given, all tests can be rephrased in terms of the standard Normal distribution with zero mean and variance one. No putations are necessary any more, but test statistics can be drawn from tables. 3. The use of accident statistics for traffic safety policy. The testing procedure described has its merits for those types of analysis that are based on the assumptions mentioned. The best example of such an application is the monitoring of safety for a country or region over a year, using the total number of accidents (eventually of a particular type, such as fatal accidents), in order to pare this number with the oute of the year before. If sequences of accidents are given over several years, then trends in the developments can be detected and accident numbers predicted for following years. Once such a trend is established, then the value for the next year or years can be predicted, together with its error bounds. Deviations from a given trend can also be tested afterwards, and new actions planned. The most famous one is carried out by Smeed 1949. We will discuss this type of accident analysis in more detail later. 1. The application of the Chisquare test for interaction is generalised to higher order classifications. Foldvary and Lane (1974), in measuring the effect of pulsory wearing of seat belts, were among the first who applied the partitioning of the total Chisquare in values for the higher order interactions of fourway tables. 2. Tests are not restricted to overall effects, but Chisquare values can be deposed regarding subhypotheses within the model. Also in the twoway table, the total Chisquare can be deposed into interaction effects of part tables. The advantage of 1. and 2. over previous situations is, that large numbers of Chisquare tests on many interrelated (sub)tables and 9 9 corresponding Chisquares were replaced by one analysis with an exact portioning of one Chisquare. 3. More attention is put to parameter estimation. ., the partitioning of the Chisquare made it possible to test for linear or quadratic restraints on the rowparameters or for discontinuities in trends. 4. The unit of analysis is generalised from counts to weighted counts. This is especially advantageous for road safety analyses, where corrections for period of time, number of road users, number of locations or number of vehicle kilometres is often necessary. The last option is not found in many statistical packages. Andersen 1977 gives an example for road safety analysis in a twoway table. A puter programme WPM, developed for this type of analysis of multiway tables, is available at SWOV (see: De Leeuw and Oppe 1976). The accident analysis at this level is not explanatory. It tries to detect safety problems that need special attention. The basic information needed consists of accident numbers, to describe the total amount of unsafety, and exposure data to calculate risks and to find situations or (groups of) road users with a high level of risk. 4. Accident analysis for research purposes. Traffic safety research is concerned with the occurrence of accidents and their consequences. Therefore, one might say that the object of research is the accident. The researchers interest however is less focused at this final oute itself, but muc