Survey models for a vehicle speed survey

Abstract: The impact of some errors, associated with a road traffic survey, is examined. The survey aims at evaluating efforts to reduce the speeds on Swedish roads. It covers both state and urban roads, but we consider only the latter. From these roads, observational sites are selected by a three-stage sampling procedure. A measurement device installed on the road is used to collect data, from which the average speed on the roads is estimated.We focus on errors in the frames used in the final sampling stage, and on errors due to rnissing data. The impact of these errors on the total error of the survey estimators is investigated. Also, possibilities to reduce the total error by weighting adjustments for missing data, and by re-allocation of the sample over sampling stages, are explored. We approach the problems partly theoretically, by use of various error models; partly empirically, by collecting data on the errors. Throughout. the sampling design of the survey is taken properly into account. We conclude that the frame error under consideration probably does not bias the estimator of average speed, and only implies a minor increase of its variance. It remains unclear if the estimator needs to be adjusted for missing data - a theoretical frame for further investigations is however provided. For unchanged total sample size, the precision of the estimator is likely to improve if the sample sizes in stage three are increased, and the sampling sizes in stage one decreased correspondingly.