Sending the right signals

The long-awaited new Guidance on Transport Assessment was finally published in March of this year, and replaces the previous 1994 Guidelines for Traffic Impact Assessment. With much emphasis on sustainability, and travel other than by car, the new guidance requires more thorough investigation and analysis, but appears to be less in favour of physical highway improvements. The inevitable question is what is best for the economics and proceeds of development.

Transport assessment and design
Following detailed analysis, a Transport Assessment will forecast the additional trips by each mode of transport. As previously the impact of extra vehicle movements is quantified in terms of congestion, particularly at road junctions in the vicinity. There may also be public transport studies to identify any shortfalls in capacity. For trips on foot or by bicycle, access points and new road crossings are examined, together with the adequacy of local footways, cycleways, and existing road crossings.

Having raised the problems of impact, the transport assessment moves on to seek solutions, called mitigation measures, but mainly outside the scope of the guidance. Because the whole process is known as ‘assessment’, it is easy to under-estimate the importance of problem-solving and design skills in developing proposals where a physical highway improvement or a busy access road is likely to be required.

Why traffic signals are important
For major developments in built up areas, a transport assessment will probably show one or more of the following:
- A nearby traffic signal or other junction would become (or be more) congested
- A nearby traffic signal or other junction is inadequate for pedestrians or cyclists
- Development access off the existing road would require a traffic signal junction
- Pedestrians and cyclists would require new or relocated traffic signal crossings
- New roads within the development require traffic signal junctions or crossings

The increasing dependence on traffic signals is due to their potential for maximising vehicle capacity on existing roads, and because other junctions or crossings, such as roundabouts and zebras, are seen as inadequate for pedestrians and cyclists.

Traffic signal modelling and design
The modelling of existing junctions and the design of new traffic signal junctions is dependent on computer methods. Since 1985 the JCT LinSig program has been the industry standard for stand-alone traffic signal junctions. For many more years, road networks with traffic signals have been modelled using the TRANSYT program from TRL. Where developments require traffic signal junctions or crossings to be modelled together as a pair or small network, TRANSYT has always been the program to use.

Following the introduction of LinSig Version Two in 2006, there is now a choice of programs for small networks, including signalled roundabouts. With many of the features of TRANSYT now incorporated, LinSig is able to model small networks with easier preparation, greater precision, and more transparent results (see screenshot below). Because LinSig is an interactive program geared to traffic signal design, it is particularly well suited to developing access junction and nearby mitigation proposals.

Best outcomes
Building new or upgraded traffic signal junctions is inevitably expensive, and it is vital that the least costly options be properly identified. However, resulting congestion at junctions or inadequate crossings may well discourage occupiers or customers, and prove even more costly in terms of the economics and proceeds of development. Transport assessments should therefore be commissioned to include the application of significant problem solving and traffic signal design skills.

For further information contact JCT Consultancy on 01522 754681, email sales@jctconsultancy.co.uk, or visit the website at www.jctconsultancy.co.uk