Safe-Speed Rule Can Be Slippery

Dear Street Smart:

Thirty or 40 years ago, Popular Science published the results of tests of cars driving in the rain. It was found that tires lose contact with the road surface at 55 mph.

Today, every time it rains, newspaper articles and television commentators advise us to “drive slow” when it rains, but they never define the meaning of “slow.” Many freeway drivers think they are driving slow at 60, but at 60 that car will very likely hydroplane. Wouldn’t it be better if the warnings were to state, “To be safe, do not drive over 50 when it’s raining”? Or, perhaps, “Stay alive, never drive 55 in the rain.”

Carl E. Joyce

South Laguna

You’re right about one thing. Cars driving with normal tread on their tires and with half an inch of water on the road can begin to lose contact with the asphalt, or hydroplane, at 55 mph. But a half-inch of rain is really a downpour.

There are three factors that determine the point at which a car’s tires lose contact with a wet road. According to Don Uelmen, regional programmer for the National Highway Traffic Safety Administration, they are: the amount of water on the road, the amount of tread on the tire and the speed of the car.

The purpose of the tread, of course, is to give the water a groove through which to shoot to the rear of the tire. As the car gains speed, however, the amount of water in front of the tire slowly builds until you reach a point where the tire skims across the top of the water just as a skier would. This is called hydroplaning, and its dangers are obvious.

Usually you can feel when your car begins to hydroplane, Uelmen says. You’ll be driving along and suddenly you experience a temporary loss of traction, as if your transmission has slipped. What ordinarily happens at that point is that, as the car begins to hydroplane, it loses momentum and drops back to the pavement again. The solution is to reduce your speed.

The NHTSA has refrained from recommending the posting of specific speed limits for rainy days because conditions can vary. “It would be like putting up a sign saying you should go a certain speed in the fog,” Uelmen said. “There are times in the fog when you should pull off the road and not drive at all.”

In general, though, he endorses your recommendation. “If people could stay at around 50 mph in a rain and have good tread on their tires, they will probably be all right.”

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Dear Street Smart:

What is the CHP’s policy regarding use of radar on freeways and highways? Recently, they have posted “speed checked by radar” signs on Interstate 5 north of the Grapevine. I thought they were limited by state law from using it on interstates. I know they use it to patrol county roads.

Also, I’ve recently seen radar jammers (not radar detectors) advertised. These actually disable the radar by causing the unit to give a “no read,” or, in some cases, to cause the radar unit to give a readout that the jammer’s user determines. Do these things actually work, and are they legal in California?

Cregg Loop

Lake Forest

Last question first. Radar jammers are so new that the California Highway Patrol has not determined whether they work. While California law does not prohibit them, federal regulations require a Federal Communications Commission license for anyone operating a device that transmits a radar signal. So jammers transmitting radar signals without a license are illegal, while those that use a laser or infrared technology are not.

The CHP has no problem with the older radar detectors that detect the presence of radar but do not interfere. “That’s fine with us because it means that people are going to slow down,” spokeswoman Anne Richards said.

Regarding your first question, Richards says that there is nothing prohibiting the CHP from using radar to detect speeders wherever it is necessary, including on the interstate. Recently, she said, the agency began posting signs over a large section of Interstate 5, including north of the Grapevine. The decision was made, she said, after surveys showed average speeds climbing in that area after the 1995 increase of the speed limit from 55 to 65 mph.

Ordinarily, Richards said, speeders are issued only warnings during a 30-day grace period beginning when the “speed checked by radar” signs are erected.

Along the stretch of interstate in question, she said, the 30-day period has not yet begun because the signs are still being put up.

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Dear Street Smart:

I thought that after officials put in the truck bypass where the El Toro Y is, the trucks would have to use them. However, I’ve noticed on my trips home on the southbound Santa Ana Freeway that some trucks ignore the bypass and instead continue going along with the normal flow of cars. Is their not using the bypass legal?

Stan Eskin

Laguna Hills

Absolutely legal. The bypass is not mandatory for trucks, said Maureena Duran-Rojas, a Caltrans spokeswoman. It is simply an option offered to keep the main line clear while allowing truckers a relatively quick and easy passage.

Mandatory bypasses exist as alternatives to hazards such as low bridges or areas with weight restrictions. This particular bypass, Duran-Rojas said, is optional because it is built at a 5% grade. “That could be a problem for some trucks,” she said.

Caltrans is conducting surveys to determine how many trucks use the bypass. Its status is indicated by signs featuring white lettering on a green background, as opposed to black lettering on a white background, which would indicate a mandatory bypass.

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Street Smart appears Mondays in The Times Orange County Edition. Readers are invited to submit comments and questions about traffic, commuting and what makes it difficult to get around in Orange County. Include simple sketches if helpful. Letters may be published in upcoming columns. Please write to David Haldane, c/o Street Smart, The Times Orange County Edition, P.O. Box 2008, Costa Mesa, CA 92626, send faxes to (714) 966-7711 or e-mail him at David.Haldane@latimes.com Include your full name, address and day and evening phone numbers. Letters may be edited, and no anonymous letters will be accepted.