Institute of Transport Studies, University of Leeds
Abstract
This paper was prepared for the Department of Transport, as a
support document to a main report on the feasibility of
measuring responses to highway improvements. The paper
discusses the statistical issues involved, particularly as
regards the determination of suitable sample sizes. Worked
examples are provided, using such data on ambient variability
and adjustment factors as were available to us. Some of the
data is included as an appendix where it was felt to be
otherwise not easily available.
The note asks two sort of questions. 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.
Three sorts of quantities are discussed:
- total flows past a point, which may be counted by loops, tubes
or manually;
- partial flows, such as a particular 0-D flow, which require
roadside interviews;
- journey times over particular links
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