He who hesitates is sometimes saved," quipped the American humorist James Thurber. Thurber's twist of the adage doesn't seem so amusing when you approach a traffic light that has just turned yellow. In fact, it comes across as pretty good advice.

By definition, a yellow light represents changing circumstances--namely, a green signal changing to red. And when you're driving, changing circumstances usually translate to indecision. Should you speed up and try to make the light? Or should you slow down and play it safe?

There's a lot riding on your choice. Surveys show that nearly three out of every four red-light violations occur within 1.5 seconds after the light has changed--presumably by drivers who thought that they could bolt through the intersection under the yellow. And you risk more than a ticket from a hidden red light camera or a lurking law enforcement officer. Every year, about 1 million crashes occur at signalized intersections in the United States, many involving drivers who made the wrong choice when facing a yellow light.

How can you make the right choice during these moments of indecision? Traffic safety experts recommend considering the following factors:

Timing. When considering whether to accelerate or stop, remember: You must clear the intersection completely--not just get past the light overhead--to go through safely. At 45 mph, for example, you'll need a full second to move across a typical intersection. That doesn't count the time required to recognize the signal and react.

How much time do you actually have? The answer depends on the intersection. According to Federal Highway Administration guidelines, the yellow interval in most traffic signals lasts from three to six seconds. Believe it or not, traffic engineers have developed elaborate equations to determine the optimal yellow interval for particular intersections down to fractions of a second, accounting for such variables as the width of the cross street, the grade of the road and the average speed of approaching vehicles. For example, roads with higher prevailing speeds generally have longer yellow lights.

In addition to timing, engineers consider the option of an "all-red" interval at certain intersections--usually, a one- to three-second period during which the traffic light glows a steady red in all four directions. The all-red interval provides additional time for the intersection to clear before cross traffic can proceed.

So much thought goes into signal timing because engineers want to guide the greatest number of cars through the intersection without creating conflicts that lead to collisions. "The selection of an appropriate yellow interval length and the decision whether to employ an all-red interval are important for both safety and capacity," notes a recent report prepared for the Institute of Transportation Engineers. "A difference of one second to the length of the yellow change interval can affect the safety of the intersection ... On the other hand, if the change interval is too long, the capacity of the intersection will be decreased and the delay to motorists will be increased. This will breed disrespect for traffic signals in general."

Ironically, longer yellow intervals may reduce serious "T-bone" crashes but increase crashes of a different sort.

"Generally, studies show that the incidence of rear-end collisions tends to go up with longer yellow lights," notes Russ Raider of the Insurance Institute for Highway Safety, a Virginia-based research group. "Longer yellow intervals increase the indecision time for motorists. They don't know whether to accelerate or stop." And the danger of a rear-ender raises another factor to consider.

Traffic environment. As you approach the intersection, check your mirror. Who is behind you, and how closely is the person following? Often, a tailgater will ride your back bumper, trying to prod you through the yellow.

"If you don't think you can make it through, tap your brakes lightly first well in advance to signal your intention," advises Frank Niland of the Club's Traffic Safety Department. "Generally, he'll back off." Unfortunately, that trick usually works only once. "If it happens again, then he'll know you won't stop at the first flash of your brake lights and he'll learn to close the gap between you," notes Niland.

Pedestrians can behave even less predictably. Before you decide to accelerate, scan the corners ahead, right and left, for people ready to step off the curb into your path ahead of the "Walk" signal.

Road conditions. Of course, wet or icy pavement increases your stopping distance. "You have to ask yourself, 'What's my stopping ability?'" notes Niland. "In poor road conditions, you need to back farther off the guy in front of you to account for longer stopping distances. And you need to reduce your speed as you approach the intersection." Otherwise, you might find yourself sailing through the yellow--whether or not you intended to.

Sneaking Left on Yellow
Police and traffic safety professionals call them "sneakers." They're drivers who want to turn left, sneak past the stop line even if the light is yellow and wait in the intersection for a clear path in which to execute the turn. You've probably done it hundreds of times. But is it legal?

"You may legally move into the intersection on either a green or a yellow light," notes Frank Niland of the Club's Traffic Safety Department. "During a heavy traffic flow, a driver wanting to turn left may creep into the intersection and wait for a break in traffic. If the light changes to red and you're still in the intersection, don't back up. Make the turn. I've never heard of anyone getting a ticket under those circumstances."

If you're already in the intersection, backing up presents a far more dangerous maneuver than completing the left turn, notes Niland. The only catch: You must enter the intersection on green or yellow when waiting to make the turn. Try to sneak left under the light as the second or third vehicle, and you risk pushing your luck--and an officer's tolerance.