Since 2010, the Ada County Highway District has installed the new signals at more than a quarter of the county's traffic-light intersections, which number 422.
Made entirely of arrows, the signals use a flashing yellow rather than the traditional green circle to indicate to drivers that they may turn left after yielding to oncoming traffic.
ACHD is pleased with the results.
"No safety issues have been detected," said spokesman Craig Quintana. "The original experiment along State Street, which received the first seven yellow arrow signals in early 2010, has been good, with no increase in crashes since the first of the yellow arrows were rolled out."
The Boise Police Department likes the new signals as well. Although the department has not been tracking the effect of the new signals statistically, officers have said they are pleased with the change, said spokesman Chuck McClure.
"The signals seem to be less confusing for motorists and the officers feel there has been a decrease in crashes," he said.
DRIVERS, ACHD MUST MAKE GOOD DECISIONS
With protected left turns - turning left on a green arrow - drivers have the right of way, but this limits the amount of time left turns can be executed, often causing traffic to back up.
Permissive left turns - turning left at a green light or flashing yellow arrow if the coast is clear - help keep traffic moving, but they also require more driver responsibility.
"This is a balance between better traffic flow and safety. It's obviously safer to only permit turns on a green when the opposing traffic has been stopped at a red light, but that slows everything down," said Quintana. "The public loves the arrows, which are more intuitive than the old-style green balls; yellow is well understood to say 'go, but be safe,' so there's less room for misunderstanding."
ACHD, too, must make good decisions when deciding where to install them.
Not all intersections are suitable for the flashing yellow arrows - as a general rule, ACHD will not install these signals on roads with dual left-turn lanes or high speed limits.
The agency also considers traffic volume, sight distance and crash history when deciding which intersections are candidates for the arrow signals.
ACHD has installed them at 112 intersections, and another 80 are candidates.
Certain locations have been requested but do not meet the proper criteria, Quintana said. He cited Eagle Road and Bristol Heights/Hobble Creek as an example - it has a 55 mph speed limit and high traffic volume.
"Part of the risk of using this technology is it will allow for some bad decisions to be made by drivers, which has made us a bit less likely to deploy them on the highest-speed roads," Quintana said.
NEW SIGNALS FLEXIBLE
After several years of testing, the Federal Highway Administration approved the flashing yellow arrows for wide use in late 2009, saying they reduce traffic delays and accidents. Since then, the signals have been installed in cities large and small.
An Oregon study earlier this year found that some drivers became so "fixated" on finding an opening in oncoming traffic to make a permissive left turn that they sometimes failed to observe pedestrians.
ACHD said it is aware of that concern but has not had any documented incidents.
This has more to do with permissive left turns in general and not the flashing yellow arrows, Quintana said.
"Motorists were allowed to make left turns while pedestrians had a walk signal long before the use of flashing yellow arrows; it just used to be a green ball where we allowed permissive left turns," he said. "Drivers still must yield to oncoming and same-direction traffic, which includes pedestrians in the crosswalk."
If an intersection becomes problematic, ACHD can remedy the problem quickly.
"A nice feature of the flashing yellow arrows is we can change it to a left-turn-on-green arrow if the need arises, say for peak times when lots of pedestrians are crossing and we need to lower the opportunity for conflict. We do that around schools," he said.
Earlier this year, ACHD took the arrow signal at Fairview Avenue and Cloverdale Road out of service at peak times. When the arrow signal was operating, Quintana said, drivers wanting to make left turns were not getting large enough gaps to complete the turn.
"Taking the arrow out of service during these peak times avoids the problem of people feeling the need to take chances because the signal is telling them it's OK," Quintana said.
Cynthia Sewell: 377-6428, Twitter: @CynthiaSewell