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Scottsdale staff defend flexible left‑turn timing and pilot detection tech; commissioners press safety tradeoffs
Summary
Scottsdale traffic engineers told the Transportation Commission they use a flexible, site‑by‑site approach to left‑turn phasing and signal timing, meeting or exceeding national guidance, and are piloting detection systems to extend greens for bicyclists and vehicles in potential dilemma zones.
Scottsdale traffic engineers described the city’s Traffic Management Center (TMC) operations and defended a flexible, location‑by‑location approach to left‑turn phasing and signal clearance timing, while outlining pilot technologies to reduce conflicts for bicyclists and vehicles in “dilemma zones.”
“Scottsdale’s Traffic Management Center…is managing traffic at over 300 intersections and more than 14,000 streetlights across the city,” said Lehi Waal, senior manager and traffic engineer. Waal told the commission the TMC adjusts signal timing daily, uses about 226 miles of fiber and more than 200 video detection systems, and seeks to balance safety and throughput with context‑sensitive timing choices.
Waal explained three left‑turn operations — permissive, protected and protected‑permissive — and recounted peer practice and guidance from NCHRP, ITE and the MUTCD. He said Scottsdale generally uses leading left‑turn arrows where they improve operations, but that the city selects lagging or mixed phasing where geometry, storage constraints or driver behavior make that safer or more efficient.
“Leading is most of the time going to optimize the operation,” Waal said, while adding that certain locations with heavy through volumes must be treated differently to avoid blocking through lanes. He also said Scottsdale’s yellow and red clearance times “meet or exceed” minimum MUTCD recommendations and that two agencies are piloting longer clearance intervals.
Traffic staff demonstrated adaptive detection pilots: a bicyclist detection box that extends green time until the last rider clears, and a dilemma‑zone detector that extends green when a vehicle is detected in a zone likely to be trapped by a yellow change. Waal said pilots will be deployed at high‑crash intersections if tests continue to show benefits.
Commissioners pressed staff on safety tradeoffs. Vice Chair Wilcoxon and Commissioner Marmon warned that widening and standardizing operations can increase speeds and that signals are not inherently traffic calming; Wilcoxon also raised the distinction between turbo and conventional multi‑lane roundabout designs in separate corridor discussion. Commissioners suggested combining detection extensions with enforcement or red‑light camera approaches to avoid rewarding risky behavior. Waal said red‑light camera policy is a police decision but noted the city can extend greens or hold phases in real time to avoid secondary crashes.
Commissioners and staff discussed resource questions — where to lock detectors, how long to hold green for platoons, and how to select corridors for deployment. Staff said one bicyclist detection location is live at Drinkwater and Fifth and four more systems have just been installed for testing. The TMC can extend greens for incidents for several minutes when needed to clear long backups.
Staff recommended continuing the flexible approach — selecting lead, lag or mixed phasing based on corridor conditions — and advancing pilot deployments for adaptive pedestrian and bicycle detection and dilemma‑zone mitigation while monitoring crash data and operations.
