Traffic Signal Design Process

There is a common belief, often from drivers that are stuck in queues, that traffic lights are installed at a junction with little consideration for the traffic flows or dominant turning movements. Whilst this may be the case for some ill conceived installations or portable traffic signals at roadworks there is however a lot of thought and preparation that goes into the traffic signal design process for new signal controlled junctions.

The reasons for installing signals can be varied and may include:

  • Allowing the opportunity for minor side road flows to enter/cross heavy main road flows

  • A desire for controlled pedestrian or cycle crossing facilities

  • To control and assist safe traffic movements where visibility is restricted

  • To reduce an accident problem

  • To accommodate an increase in traffic associated with an adjacent development

The processes set out below are the same for either signal controlled or priority junctions, however this document considers signal controlled junctions only.

Identification of a development scheme

  • Is it a redevelopment of an existing site or a complete new build on a new site?

  • Will new/additional access points be provided?

Identification of potentially affected junctions

  • Are there any nearby junctions that will be affected by the additional traffic from the new development?

  • Do pedestrian or cycle facilities need incorporating into an existing junction?

This will depend on the type of development and the associated levels of generated traffic.

Undertake traffic modelling

Use sophisticated software to predict the impact on any affected junctions and new accesses to the site.  It may be necessary to assess priority controlled junctions which may need upgrading to traffic signal control.

Analysis of traffic modelling

Results will show if improvement works will be required – this could be the introduction of traffic signals at a previously priority controlled junction, localised widening at an existing junction to provide an extra lane, or the introduction of controlled pedestrian crossing facilities either within a junction or as a standalone pedestrian crossing.

Detailed design

Create full designs for proposed improvements or new junctions identified through the modelling, it may be necessary to undertake further modelling to ensure designs will operate satisfactorily for a number of additional traffic flow scenarios. The full detailed design will comprise a suite of drawings including kerbing, surfacing and the above and below ground traffic signal details (i.e. traffic signal pole locations and the underground ducting routes).

Produce controller specification

The controller specification document forms the basis of how traffic signals will work on street and contains the timing and safety information.

Test controller operation off street

All traffic signal controllers must be tested before installation on street to remove any problems or errors. This is done using a simulation of the junction either on a computer or in a lab with a small scale mock up of the junction. Any issues identified can be rectified and re-tested before on site installation.

Installation of signals on street

Specialist contractors will install traffic signals on street. The ducting system for the cables will have been previously installed by a civil engineering contractor as part of the overall works on the scheme.

Test controller operation on street

Switch on new traffic signals outside of peak traffic periods and run a series of tests/checks to ensure controller is working as expected.

Final adjustments

Make any necessary changes on street for example to timings and connect to central computer network where applicable.

On going monitoring

Frequent observations (usually undertaken by Local Highway Authority) to ensure the junction(s) are working as well as possible and making any changes to timings etc if necessary.

The above design process may be undertaken by either a Local Highway Authority or a private developer/contractor.  As the Local Highway Authority have responsibility for the operation of traffic signals then any private developer/contractor must have full agreement of the Local Highway Authority for any works to existing or new traffic signals.

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Traffic Signal Design Process, Identification of Scheme, Affected Junctions, Traffic ModellingDetailed Design, Controller Specification, Test Controller Installation of Signals on Street, Final Adjustments, Ongoing Monitoring