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Annoying lags often key to traffic signal timing

Dr. Delay Navigating Pinellas

By LORRIE LYKINS, Dr. Delay
Published August 2, 2007


Traffic signal timing is always a hot topic for Dr. Delay readers. Here's a question from Scott Hood about signals on some area roads:

"Why are the lights staggered for the opposing traffic here in Pinellas County at four-way intersections? The reason I ask is that some intersections have a second or two delays, and others seem to be on a 15- to 20-second delay for no apparent reason."

Hood said the intersection with this odd timing that bugs him the most is Ulmerton Road and 66th Street. Another puzzle is intersections that have turn lights that only allow left turns from one direction at a time rather than allowing, say, cars in the northbound left turn lane to turn on a green light at the same time as cars in the opposing southbound lane turn on a green arrow.

We checked in with Ken Jacobs, traffic signal operations manager for Pinellas County to address these questions.

Jacobs said the primary objective of signal timing is to safely assign the right of way to alternating flows of traffic, providing for orderly movement of traffic between intersections.

Traffic signal simulation models are used by the county's signal operations folks to analyze and emulate traffic movements along the major roads, specifically to reduce delays and provide "green to green indications at adjacent intersections" synchronization. Jacobs said that main street traffic flow gets priority, so when crossing a road designated as a main street there's potential for the wait to be longer.

The staggered left-turn green signals is what's called "lead-lag," Jacobs added.

This model of traffic management improves the progression of traffic between intersections by sequencing the left turns to correspond to the arrival of traffic coming from adjacent intersections.

"This operation is in use at the intersection of Ulmerton Road and 66th Street. Since the volume of traffic on Ulmerton Road is significantly greater than the traffic on 66th Street, the green-to-red time and left-turn movements are designed to maximize the movement of traffic on Ulmerton Road and minimize the delay on 66th Street," Jacobs said.

GANDY BLVD. AND I-275

Posted ramp speed is intended for safety

Reader Sue MacFarlane is wondering about one of my favorite subjects: ramp speed. She wrote:

"Why when traveling east on Gandy Boulevard, turning onto Interstate 275 south, is there a sign that reads Ramp Speed 35 mph? Some people slow down to that speed and, during rush hour in particular, cause the drivers behind them to hit their brakes."

We asked the Florida Department of Transportation about the ramp speed signs. Kevin Dunn, DOT's district signing and pavement marking manager, said it's a safety measure.

The ramp speed is set at 35 mph because of the degree of curvature on both the southbound and northbound sides of the ramp, as well as on the northbound flyover ramp.

"The original design recommended 35 mph as a safe speed to travel," Dunn said.

The posted speed on the ramp is not a regulatory speed limit, so if you hit the ramp at 40 mph you wouldn't expect to receive a speeding ticket, he said.

The posted speed is a suggested safe travel speed to allow for negotiating the ramp under worst conditions, such as heavy rain. Safe speed recommendations are determined by how much centrifugal force should be applied to a vehicle to negotiate a given curve safely.

"My experience has shown that many drivers do not reduce their speed based on these warning sign recommendations, so it was nice to hear that some are viewing and reacting to these signs as intended," Dunn said.

Lorrie Lykins' Dr. Delay column runs regularly in the south Pinellas editions of the Times. Please share your traffic concerns, comments and questions with Dr. Delay via e-mail at docdelay@yahoo.com.