The popular recent article on the BBC website, “Does pressing the pedestrian crossing button actually do anything?” was accurate with regard to the facts, having been well researched.
The follow up to the article was significant, with many people having a strongly held opinion on the function of push buttons such as whether machines should be allowed to make decisions instead of humans, whether people have to wait too long or whether pushing the button at all or too many times makes any difference at all.
For a general interest article it was technical enough without being too “geeky”. This article aims to provide further technical details for those with an interest in this subject and takes a particular look at the use of push buttons in a SCOOT region.
SCOOT stands for Split Cycle Offset Optimisation Technique. It is a software tool within the Urban Traffic Control (UTC) system which was recently described as a super computer on the BBC programme “The Route Masters” which followed the many roles of Transport for London. UTC is a system which is able to coordinate sets of signals using fixed time plans based upon historic information. SCOOT undertakes a similar function, however through the use of detectors it makes decisions for optimising the green times based upon real time traffic flow information.
SCOOT is a widely used tool that is proven to provide greater than a 20% reduction
in delay over other existing systems. The Green Signals team has significant experience configuring SCOOT on road networks both in the UK and overseas and can speak first hand of observing noticeable benefits to traffic flow as a result of optimal configuration of the system. Informal journey times taken on some regions have proven significantly higher than 20% reductions with one recent region on average having been improved by approximately 60%.
When configuring SCOOT the Engineer has to consider the maximum cycle time for every junction which requires coordination within a region. A function and also a limitation of SCOOT is that all signals in a coordinated region must run the same cycle time. Normally in the UK the maximum cycle time is 120 seconds, and within this time all green signals must appear at least once; however the duration of each green will vary depending on traffic demand. The cycle time also includes the “intergreen” which is the safety clearance time between conflicting green signals (ie where no green signal is shown on any approach at a junction).
The Engineer will also need to consider the minimum cycle time, where yet again all green signals (including the pedestrian green man) and intergreens are considered. The normal minimum green time for traffic phases is 7 seconds and average green man time is 6 seconds. Intergreens are often in the range of 5 to 8 seconds (dependent upon the geometry of the junction), with the intergreen from a pedestrian green man normally longer, in order to provide a safe duration for the pedestrian to cross.
The main criticism of push buttons following the BBC article was that they often do not need to be pushed in order to call the pedestrian phase, however, “walk with traffic” for non-conflicting pedestrian movements is a more efficient technique for operating the junction as it can be run at the same time as flowing traffic and in a safe way. In a SCOOT scenario, the pedestrian phase must be introduced every cycle in order to maintain the balance of traffic coordination across the region. Should the crossings be activated by
human intervention only under this scenario then vehicle coordination would be lost and the 20% reduction in delay for vehicles would be significantly reduced.
Whilst it is understandable that pedestrians may become frustrated at having to wait for up to 2 minutes to cross a road, this should be a worst case scenario. The job of the Traffic Engineer is to balance the needs of all users. SCOOT is configured to vary the cycle time as the volumes of traffic change, therefore as the roads get busier, the cycle time gets longer
and pedestrian crossing times increase. As traffic reduces, the cycle time reduces and pedestrian wait times will also reduce. This is a reasonable compromise for all parties. If road networks were made to only favour the pedestrian, this would quickly increase congestion for vehicles, causing pollution, delay and driver frustration.
The key to sensible implementation of any traffic system is to ensure the right balance is struck between all road users. This can often be a subjective issue and may depend on the political agenda of the time. The holy-grail for the Traffic Engineer is to obtain the right balance and keep all users satisfied.
Three general points to note when using a crossing.
- A green man displayed at a crossing or junction does not mean it is safe to cross. Cars do occasionally jump red lights, especially if the driver is distracted. Pedestrians should always check the road is safe to cross, do not assume vehicles will stop.
- There is an inbuilt safety feature that if one red light stops working the intergreen to the green man will be increased. If two red lights stop working the green man will be prevented and the red man and wait light will stay on. If you notice the pedestrian facility never being called, please report this as a fault to your local highway authority.
- It makes no difference how many times you press the button, the signals will not change any faster.