‘Yellow trap’ and oncoming emergency vehiclesNovember 28th, 2012 by travis baker
The in basket: Rob Shafer of Yukon Harbor in South Kitsap writes, “The other day I was traveling west on Mile Hill Drive approaching Woods Road when an ambulance approached from the other direction.
“It appeared that all of the through traffic lights had been triggered to red as cars were stopped in both directions and my light was red. However, the left turn arrow on Mile Hill was still flashing yellow where an inattentive driver could make a turn directly in front of the emergency vehicle.
“Is there some fault with the emergency system where it does not turn all lights red on the approach of the emergency vehicle? This is not the first time I have seen this,” he said.
The out basket: Jeff Shea, traffic engineer in Kitsap County Pubic Works,
says the signal was working as intended and explains what Rob has seen.
“Emergency vehicle drivers are trained to watch out for motorists that make illegal movements, such as turning in front of an oncoming vehicle on a flashing yellow arrow,” Jeff said. “We held several discussions with emergency vehicle operators to go over this situation and make them aware of what the signal configuration will (be).
“When a signal is ‘preempted’ for an emergency vehicle the whole intersection does not go red. The direction the vehicle is traveling will go green while the side and opposing traffic will get red indications, except for the flashing yellow arrow in the opposing direction.
“The reason the flashing yellow arrow continues to flash is to avoid what is called the ‘yellow trap.’ The yellow trap occurs when a left turning vehicle with a green ball or yellow flashing arrow gets a steady yellow ball and assumes that oncoming traffic is also getting the yellow ball and turns in front of that vehicle assuming that it is coming to stop for the upcoming red light. This condition caused so many collisions that the federal guidance manual, the Manual on Uniform Traffic Control Devices, specifically restricts agencies from configuring signal systems to allow this.
“Therefore, we continue the flashing yellow arrow, and hope that motorists obey both the legal requirements not turn in front of traffic, or an emergency vehicle with its lights and sirens on.
“There is another option which requires us to terminate all the movements that are going at the time of preemption. In this situation, also, the MUTCD is very clear that we cannot shorten any of the clearance times – the yellow and red times.
“So, by the time all those clearance times are complete and the green ball comes back up for the preempted direction the emergency vehicle may already be at or beyond the intersection making the preemption. We felt this was not a good option.”