July 4, 2010

Stop or go on yellow? Many factors go into decision

Drivers know that green means go, red means stop and yellow means . . . "Can I make it?"

Drivers know that green means go, red means stop and yellow means . . . “Can I make it?”

Although the law is clear that yellow means slow down and prepare to stop, many drivers do not. New research sheds light on what factors come into play when a driver decides to run those yellows, and it turns out it’s not just a matter of speed.

Researchers from the University of Cincinnati, funded by the Ohio Department of Transportation, monitored four “high-speed” traffic intersections in suburban areas of Ohio, using video cameras to track more than 1,500 drivers. They measured vehicle type, speed, driver’s distance from the light when it turned yellow, and decision to stop or not in what they called the “dilemma zone.”

Wichita traffic engineers said the research probably won’t change the way the city’s 400-plus traffic signals operate.

But the “dilemma zone” concept is exactly what prompted the city to install “countdown” crosswalk lights at intersections across the city.

“The same theories apply to pedestrians,” assistant city traffic engineer Brian Coon said.

Like drivers, he said, pedestrians sometimes have to make a quick decision about whether to risk crossing a street when the crossing light has just switched from “walk” to “don’t walk.”

The Ohio researchers found that cars traveling in right-hand lanes tended to go through yellow lights, while those on the left did not.

Truckers also tended to speed through yellows, as did drivers on streets with higher posted speed limits. Drivers of vehicles on streets marked by 55 mph speed limits were more likely to run yellows than those in 50 mph zones.

The researchers found that drivers of SUVs, pickups, sedans and vans tended to slow down at yellows more than drivers of heavy trucks. They speculated that vehicle weight may be the explanation, since heavy trucks have more difficulty decelerating rapidly than other vehicles.

Not surprisingly, how long the light remains yellow also matters. (Yellow-light times vary, but typically last about three to five seconds. Traffic engineers base the time on the average speed of the vehicles passing through the intersection.)

Coon, one of Wichita’s traffic engineers, said it’s not hard to understand the concept.

Envision a scenario, he said, where you approach a yellow light from a quarter-mile away, then notice it’s still yellow when you’re 400 yards away, and then notice it’s still yellow when you’re 200 feet away.

“You’re probably going to through it,” he said.

Conversely, a light that stays yellow for only a second or two can make it impossible for some drivers to avoid running a red light.

The basic rule-of-thumb, he said, one second for every 10 mph of the speed limit.

Wichita traffic lights

In downtown Wichita, where the speed limit is 30, lights usually stay yellow for three seconds. On the fringes of the city, where speeds are 40 or 50 mph, yellow lights stay on longer.

A similar philosophy is used in determining an intersection’s “all-red” time — the amount of time at the end of a cycle that lights shine red in all directions. The system is designed to provide a cushion that accounts for drivers who push the envelope at either end of their cycle.

Longer all-red times don’t necessarily translate to safer intersections, Coon said.

When a long all-red time is programmed at an intersection, he said, it doesn’t take drivers long to realize that they have an extra two or three seconds to get through it. That might prompt some to run yellow lights they normally would stop for.

Downtown intersections in Wichita typically have a one or 1½-second all-red time. Larger intersections, such as the one at 21st and Tyler, need longer all-red times to clear the intersection of traffic between light changes.

Coon said the rule-of-thumb for pedestrians is 3.5 feet per second of crossing time. For a 50-foot wide street, that’s 14.3 seconds.

The “countdown” lights that are being installed throughout the city are designed to remove the “dilemma zone” for pedestrians and tell them exactly how much time they have to clear an intersection.

With most lights, the green light switches to yellow the instant the countdown clock reaches zero. There are some intersections, however, where the yellow doesn’t come on until a few seconds after the countdown clock expires.

Coon said that’s because some intersections operate on cycles that change during peak traffic hours — say the morning and evening rush hours.

When an intersection uses such a split schedule, he said, the crosswalk light will reflect the shortest amount of time pedestrians have to get across that intersection at any point during the day.

“It defaults to safest timing,” he said.

Vehicle types

Coon said he wasn’t surprised that the Ohio researchers found that drivers of trucks were less likely to stop for yellow lights than drivers of sedans and pickups. But he questioned the notion that weight is the main consideration.

Truck drivers typically are professionals, he said, and for them time means money.

“They’re more goal-oriented drivers, rather than the driver who’s just driving to pick up something at Home Depot,” Coon said.

Wichita police records don’t suggest that truck drivers are more likely than others to run red lights.

Of the tickets issued by police for running red lights from 2004 through 2009, 0.5 percent went to big trucks — a figure comparable to the percentage of all tickets that went to such vehicles.

Truck drivers, by comparison, received 0.9 percent of inattentive driving citations and 1.0 percent of stop sign tickets.

The Wichita data suggests that SUV drivers may be the most likely to run red lights. SUV drivers accounted for 6.8 percent of all citations, but 8.1 percent of citations for running red lights.

Coon said other studies have shown that drivers of SUVs are more likely to engage in risky on-road behavior such as tailgating and running stop lights.

In general, Coon said, you can figure that 5æpercent of drivers in any kind of vehicle will be the ultra-cautious type who stops at the first hint of a yellow light. Another 5 percent, he said, will be risk-takers who push the envelope of all traffic laws.

He said keeping lights tuned, with the proper amount of yellow and proper amount of all-red, can make streets safer for all drivers.

“We don’t ever want anyone to be surprised,” he said.

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