Petaluma drivers may soon experience a strange new traffic signal — a flashing yellow turn arrow.
Traffic officers have detected an unusual rise over the past three years in left-turn crashes, part of a category of accidents known as "right-of-way collisions" in traffic parlance.
City statistics from 2010 showed a first: right-of-way collisions have become the main factor in most of Petaluma's crashes.
"For as long as anyone can remember in our unit or our department, the primary collision factor has always been speed," said traffic officer Ron Klein.
He said that is also the case throughout California and the rest of the nation.
In Petaluma, most of the right-of-way crashes were at intersections with a specific type of traffic light: a protected-permissive left-turn signal, where a driver is still allowed to turn left after the "protected" green arrow turns to a regular, "permissive" green light.
"It opens the possibility for a driver to make last-minute decisions that can cause an accident," said Larry Zimmer, the city's capital improvements manager.
Most of Petaluma's protected-permissive signals are at intersections that have speed limits of 35 mph or more, Klein said. Collisions at those speeds can be fatal and about half of them involve injuries, he said.
So police sought and received a $441,000 state traffic grant that will fund 90 percent of a project to replace those signals deemed problematic based on collision numbers, speeds, line of sight and other characteristics.
The city will replace all or some of the protected-permissive signals at 26 intersections throughout town, while allowing a few to remain as is.
Some will include a signal new to California and what may be a first for Sonoma County — the flashing yellow left-turn arrow.
Petaluma Boulevard North at Magnolia Avenue/Payran Street had a "higher than expected collision experience involving left-turning vehicles," Zimmer said.
In all four directions, the protected-permissive signals will be replaced with protected left-turn phasing, meaning left-turn drivers will see a progression of green, yellow and red arrows.
Then, once the intersection is cleared, a flashing yellow arrow will light, alerting drivers that they may proceed with caution with a left turn when oncoming traffic is clear.
East Washington Street at Lakeville Street was identified as another trouble spot, with complications brought on because of the railroad tracks that pass through its western leg.
The protected-permissive left turn lights on East Washington will be changed to protected-only lights.
Allan Tilton, a traffic engineer with Whitlock and Wineberger in Santa Rosa, said there will be a slight learning curve for drivers seeing this new flashing yellow signal.
The flashing yellow arrow is already used in several Western states. California has also allowed it as a provisional device, and Petaluma's project should be allowed to proceed after the city notifies Caltrans, Tilton said.
He said the green arrow and green ball signals are confusing for drivers wanting to turn left while trying to judge oncoming traffic.