The topics of journey time reliability (JTR) and its evaluation in the literature were compared with current advice in Scottish Transport Appraisal Guidance (STAG). The latter seemed well up to date with the state of the art, and no major revisions were suggested. Both in STAG and in the literature, the accepted way of encapsulating the value of JTR changes is via the measure known as the reliability ratio (RR). This is discussed in some detail in the report. It has some weaknesses: it can be difficult to calculate, and can only be transported to other times and locations with great care. RR values have been calculated for both the freight and non-freight samples.
The results from the study will form part of the evidence for the next review of Scottish Transport Appraisal Guidance (STAG)
- Views of companies, both involved in freight movement and not, were obtained via surveys.
- Trunk road reliability in Scotland was perceived to have deteriorated in recent years, but this did not appear to be a matter of great concern to most companies responding.
- Analysis of Automatic Traffic data Collection figures showed a range of journey time variability by month and detection site, but generally the journey time variability did not seem particularly great.
- An approximately linear relationship was found between the standard deviation of average speeds, for days taken at monthly intervals, and the inverse of speed. Such a relationship might prove useful in forecasting journey time variability.