Keeping traffic moving by using flashing amber at traffic signals

There are around 10,000 signalised road junctions in the UK of which 2,500 are in London and the number is estimated to be growing by 3% each year. This has led to the growing perception that there are perhaps too many traffic signals, and at the margins their benefits may be outweighed by unnecessary delays – especially outside peak hours.

A study by Colin Buchanan for the Greater London Authority analysed the economic impact of using traffic signals to manage traffic at junctions in London. This was done by assessing the impact of switching off traffic signals, against retaining them, at five junctions in different areas of London using actual 24 hour traffic data. The traffic signal timings were optimised to maximise traffic throughput before the analysis was undertaken.

The key findings of the study were:

  • Overall, there are significant benefits to road users in London from installing traffic signals at junctions.  Whilst it is difficult to extrapolate from the sample junctions to the whole of London or the UK, a very rough estimate is that total annual benefits in journey time savings alone are in the order of £200m for London and £500m for the UK.  However most of those benefits accrue during the morning and evening peak periods


  • The study also found that there are clearly times of day or locations where it would be beneficial to switch off traffic signals.  For instance, all modelled junctions showed a benefit from switching off traffic signals at night. The total benefit of switching off signals in terms of reducing journey times when they are not required could be in the order of £50m a year in London and at least double this for the UK.

Based on this work Transport for London are considering removing traffic signals at a number of signalled traffic junctions. However, this raises concerns about pedestrian safety especially for vulnerable road users.

At present, highway authorities only have a choice between having signals switched on or off. In a number of continental countries it is common practice to switch all traffic signals at a junction to flashing amber at off-peak times. The flashing amber signal advises motorists that they may need to give way at the junction ahead but they need not stop if there is no conflicting movement. It also allows pedestrians to activate the pedestrian phase on the signals and therefore cross at a light protected crossing if required. In our view, the introduction of flashing amber signals at night or at off-peak times (depending on circumstances) would provide clear economic benefits in terms of time savings and help to smooth traffic flow. It is a measure that would be relatively low cost to implement, have widespread support from the motoring public – the AA, for example, have stated they are in favour – and can be done in a way that does not cause disbenefits to pedestrians.

The Department’s present view is that flashing amber signals have a specific legal meaning at pelican crossings and therefore cannot be used elsewhere without changes in secondary legislation which would take time and money given the legal complexities. In addition modern signals should be traffic responsive and therefore not cause any delay to traffic. Our analysis showed that this is not the case and this is probably even more so outside London.

However, we understand the Department is looking into various options for signalling in periods of low demand and we await the outcome with interest.



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