Improving the evidence base on journey time reliability on the Trunk Road Network in Scotland
2. Aims and Objectives
The requirements of the research were set out in the Tender Document published by Transport Scotland on 11 October 2013. The principal aim of this research was to generate first hand evidence from road users in Scotland on journey time reliability. Further aims were to assess the usefulness of the data that exists within and outwith Transport Scotland on journey time reliability and, together with the principal aim, evaluate how this information might be used to update and improve the appraisal guidance on assessing journey time reliability impacts in Scotland.
In furtherance of those aims the following key objectives were targeted:
- Bring together the latest evidence and understanding of current patterns of journey times (not limited to Scotland) along with likely causes of unreliability and the impact this uncertainty has on businesses;
- Capture relevant first hand journey time data and information from businesses using the trunk road network in Scotland;
This objective was achieved by a two part survey of road users. The initial approach was to contacted trade bodies, and similar, covering a range of activities within Scotland, and obtain a list of persons to contact by phone. Unfortunately, very few of those people contacted declared themselves available to take part in the survey at that time, so it was decided to move to the first fall-back position, the provision of an on-line version, to be filled in without the help and guidance of interview staff. Gratifyingly, the response to the on-line survey was quite good, both in terms of quality and number. However, with the responses somewhat clustered as regards activity sector, and still insufficient in number, it was decided additionally to buy in to a commercially available pre-recruited regular survey panel to fulfil the data requirement. The key results are presented in section 5.
- Review the usefulness of available journey time reliability data (data sources within Transport and Traffic Scotland and any potentially useful external data sources).
Having received early returns from Transport Scotland staff, Moving Observer data was first considered. This involves noting the times at which points are passed when making (repeated) trips along specific roads, and making well known adjustments for the numbers of vehicles 'overtaken' and 'overtaken by' in each road section. A good estimate of the underlying speed of traffic on those sections at the time of the run can be obtained. However, on further investigation it became apparent that only a small number of locations were covered and it appeared that generally only raw data was available, requiring a great deal of analysis to be performed (even if the data on overtakens and overtakers could be found), which ruled out this route of enquiry.
Turning to large data bases, such as those generated by automatic data capture, the main data set is held in the Scottish Roads Traffic Database (SRTDb) and revolves around traffic counts. With the help of the relevant team within Traffic Scotland, it was possible to get access to the raw Vehicle By Vehicle (VBV) data for a limited number of sites. Having access to the raw data was excellent but, as different sites had this data coded in several different ways, data handling was time consuming. Resource constraints restricted the analysis to one day's data for each month of 2013 for 38 sites. Results based on the 31 sites with usable data are reported in Section 6.
- Consider the implications of findings for appraisal guidance on journey time reliability.
Consideration of how the findings from this work might be reflected in STAG guidance are reported in Section 7.