While there are obvious benefits from monitoring the impact of highway improvements, it is not clear that such monitoring is worth the cost. In order to determine cost-effective monitoring procedures it is necessary to understand the relationship between sample sizes and the level of accuracy obtained. Two sorts of questions are addressed in this paper. Firstly, what is the minimum sample size to take to be a certain percent confident that a given "quantity", lies in a range of a given width. Secondly, what sample sizes should be taken in Before and After Studies so as to be a certain percent confident that a change in a "quantity" by a given amount will be detected as a statistically significant difference at some chosen significance level. The sorts of "quantities" discussed are: (1) total flows past a point, (2) partial flows (requiring roadside interviews) and (3)journey times over particular links. (A)
Abstract