Posted on: November 12, 2019 at 3:17 PM    

Many serious and fatal crashes in urban areas occur at traffic signals.  Despite this, traffic signals are usually the preferred control type at major intersections on arterial roads when there are a mix of road users. Often, exclusive right turn phases are added, or left turn slip lanes are removed to improve safety. This may improve safety for some users. However, research shows that improving safety for one road user may make an intersection less safe for other road users. 

So how do we make a step change in safety at traffic signals? There are two key factors that should targeted: operating speeds and signal violations by all road users.

Under a Safe System approach, operating speeds should be lowered to survivable levels. This includes reducing impact speeds to 30km/h or lower where pedestrians, cyclists and motorcyclists (vulnerable road users) are involved, particularly at locations and times of the day where there are higher volumes of these users.

Signal violations also contribute to crashes. Many violations are attributable to how the traffic signals are designed and operated. Filtered right turns lead to drivers waiting in the middle of the intersection until there is a big enough gap in the oncoming traffic to turn right. Often this gap doesn’t appear until the amber or red phase, putting vehicle users at risk of collision. High approach speeds can result in drivers travelling through the intersection on the late-amber or red phase, likewise long cycle times encourages drivers to take risks to avoid being delayed. These violations form a habitual behaviour pattern in some drivers.

Raised platforms are promising treatment that is being trialled across Australasia to reduce speeds and signal violations at intersections. These raised and textured surfaces slow drivers as they navigate the transition (ramp) up to the platform. Drivers are more cautious and therefore less likely to enter the intersection on a late-amber or red light.

Operating speeds can be reduced through speed limit reductions and enforcement. Signal violations and speed can also be managed using red light cameras and speed cameras, and by extending the all-red phase. Variable speed limits at traffic signals during high usage of VRUs periods is encouraged with a preference for 30km/h (or 40km/h) operating speed/speed limit. This could either be applied along an arterial route through several set of traffic signals or activated on demand at isolated traffic signals, as currently occurs at high risk priority intersections (the RIAWS treatment).

An innovative new treatment is the “Hold the Red” crash avoidance system, which is being successfully being applied in Queensland to predict when a vehicle approaching an intersection cannot stop in time, and extend the all-red phase to reduce the risk of collision.

Pedestrians are also more likely to violate the signals at the intersection or mid-block locations when the wait time is high, particularly when traffic volumes are light or moderate (Turner et al. 2006). Reducing pedestrian delay would reduce the likelihood of non-compliance and improve safety for these users. New Zealand research indicates pedestrian delay can be reduced by using off-peak signal plans with shorter cycle times, with often little impact on motorists.

While the widespread application of interventions that reduce speeds through traffic signals are most desirable, this may not be achievable in the near term.  Intersections with high volumes of vulnerable road users should be targeted first, for example central city areas. Exclusive pedestrian phases can also be used to reduce pedestrian delays in these areas. In other locations enforcement and interventions that reduce waiting times (by looking at per person delays rather than just motor-vehicle delay) can be applied.  In lower pedestrian demand areas advanced pedestrian detection could be used to activate shorter cycle times and associated crossing delays and either lower speeds or provide warning to motorists.

In summary, achieving a step change in safety at urban traffic signals requires a tailored approach, depending on the mix of users involved. Interventions that could significantly improve safety in many contexts are:

  1. Reducing operating speeds through design, speed limit reduction and enforcement.
  2. Reducing red light running violations (and its impact) through improved intersection design, enforcement and all-red extensions.
  3. Reducing delays for pedestrians by optimising traffic signals on per-person delay and by using different signal control plans across the day and week. 

Blog written by Shane Turner, Technical Director, Road Safety

iStock 624460286