Review of the Red to Green Sequence at Traffic Signals
A Maxwell, I York, TRL, UK
This paper reviews the safety and efficiency implications of traffic signals changing to green directly from red compared with the current UK policy of showing a red-with-amber before green.
The UK Department for Transport commissioned TRL to review the omission or reduction of the ?starting-amber? (the red-with-amber shown before the green) in the traffic signal sequence, both in efficiency and safety terms, compared with the current policy.
This abstract gives an overview of the project and some preliminary findings. Note that any views expressed are not necessarily those of the Department for Transport.
The red-with-amber signal denotes an impending change to green, but conveys the same prohibition as the red signal. In the UK the length of the period is fixed at two seconds. Practice varies across Europe with some countries using a starting amber and some not. The period for which the starting amber is shown also varies by country, most use either one or two seconds. Some European countries have or are considering reducing the length of the starting amber.
The main strands of the research were:
? An initial review including an international literature review, previous accident studies at traffic signals, practice in other countries, stakeholder interviews and questionnaires, and an initial assessment of the potential implications on road users (including pedestrians).
? Comprehensive video analysis of the current UK situation, recording vehicle and cyclist movements in relation to the starting amber, conflict analysis, and investigating how pedestrians use the starting amber.
? A pedestrian questionnaire, used to ascertain their perceived crossing behaviour at traffic signals and their views on a change to the starting amber.
? A TRL Driving Simulator experiment examining driver?s responses to 0, 1 and 2 second starting-amber periods. Drivers? decisions were investigated when they were approaching, or already stationary at the junction. At some of the junctions the drivers could just see the signal shown by the opposing signal heads. Drivers completed questionnaires before and after the trials.
Summary of main results
The video surveys showed that current vehicular starting delay with the two second starting amber is low. On average the front vehicle started to move 0.25 seconds before green and crossed the stopline 0.38 seconds after the green.
There was a tendency for a proportion of drivers to treat the starting amber as a signal to go. Just over a third of vehicles that waited on or behind the stopline had crossed the stopline (front wheel) before the start of green, and 4 and 17 percent after 1 and 1.5 seconds starting amber respectively. Motorcyclists had the highest rates with approximately three quarters crossing the stopline before green.
At junctions the front vehicle moved an average of about one metre and was over three quarters of a metre in advance of the stopline by the start of green. However, drivers tended to treat the situation with more caution when there was a potential conflict, or poor intervisibility with opposing movements. There was no indication that the current recommended UK intergreen times are insufficient given current driver behaviour in the starting amber. And investigations of previous accident studies at traffic signals did not identify a safety related problem associated with the starting amber.
The driving simulator experiment indicated that omitting the two second starting amber would increase the time relative to the start of green when vehicles cross the stopline by an average of 1.6 seconds for younger drivers (less than 55 years old) and 1.2 seconds for older drivers. The difference in ages appears to be largely due to more younger drivers starting to move in the red-with-amber when it was set to 2 seconds. The extra delays observed align with the findings of the initial review based upon previous trials and observations overseas. The ability to view opposing signal heads was found to be a significant determinate of the delay both in previous trials and in this. This behaviour could be of concern if the drivers became highly dependent on the information in making their decisions on the road.
Reducing the amber to one second was found to increase the average time for younger drivers crossing the stopline by about 0.7 seconds.
The results were similar for vehicles approaching the stopline and indicated that the starting amber can be beneficial to maintain progression at closely spaced signals.
These increases in starting delay would tend to reduce junction capacity if existing intergreen times are maintained (the consequences for intergreen times are yet to be assessed). For example, taking the results from the Driver Simulator, the capacity of a junction with three traffic stages operating on a fixed 80 second cycle time would reduce by 3 percent for a one second reduction and 6 percent for omission. This could be critical for junctions running close to capacity.
Driver questionnaires indicated that the removal of the starting amber results in a signal change that is easier to understand, but made drivers feel more hurried in their decisions, which could have safety implications.
Pedestrians consider that they would feel less safe if the starting amber was removed and would show more caution and request pedestrian phases more often. This could possibly cause greater delays to pedestrians and traffic, but could have a positive influence on pedestrian safety.
It is likely that the implementation costs of any change in the starting amber period in the UK would be significant if undertaken in the short term. One potential way to implement any amendment would be to require manufacturers to specify a configurable starting amber period, while maintaining the current two-seconds. Thus readily facilitating a change in the long term.
The initial results indicate that a slightly shorter starting amber period could reduce starting amber violations without overly affecting capacity. Omitting the starting amber would reduce junction capacity. This could be partially offset by a reduction in the intergreen timings. But there are concerns that drivers may be more likely to try and anticipate the start of green, which could have safety implications. Given that there is not an established safety problem with the current policy and the costs involved, it seems likely that the current starting amber policy would be maintained.
Association for European Transport