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Slow down when driving in fog, say CHP and Caltrans

Slow down when driving in fog, say CHP and Caltrans

Drivers contend with foggy conditions on their way to work and school Friday morning.

POSTED January 16, 2010 12:31 a.m.
Fog is almost synonymous with “winter weather” in the Central Valley. The area is known for its seasonal covering of dense “Tule fog” that can blanket the valley for days at a time. This unique weather condition can contribute to motor vehicle collisions both on surface streets and on the highway.
Tule fog forms on clear, moist nights. It is caused by a temperature inversion. Cold air settles at the valley basin and is covered by a layer of warm air above. Cold air in the valley is trapped by the surrounding mountains, and fog often will stay in place until a significant wind or heating of the surface dissipates it.
“Cool moist air at the surface really has no place to go,” said David Spector, a meteorologist with the San Joaquin Valley Weather Forecast Office.
Fog usually forms in the Central and San Joaquin valleys from November to January. Spector said that while there have been some foggy days this winter, he has seen at least two years out of the last 10 where the fog was considerably worse.
Spector said that once a layer of fog forms, it can often become patchy. Patchy fog can be dense in some areas, with thin fog or clear areas in between. Spector said that patchy fog can cause some problems for drivers on the I-5 and the 99.
“The particular danger for driving is when people speed up in the clear areas,” Spector said.
The California Highway Patrol and Caltrans are warning motorists to be careful when driving in the fog. CHP Officer Eric Parsons said that rear-end collisions are the most common type of traffic incident when fog is a factor. He said that although the maximum speed limit on Highway 99 is 65 miles per hour, some conditions require a lower speed.
“Fog might be a contributing factor in a crash, but it isn’t the cause. If everyone were to reduce their speed accordingly, those rear-end collisions wouldn’t happen,” Parsons said.
Caltrans and the CHP maintain a Web site dedicated to fog safety on Central Valley roadways. The Web site,, gives safety tips for driving in low visibility conditions.
The site advises motorists to slow down and use headlights on low-beam setting in the fog. Parsons said that his number one fog safety tip is “never use the high beams in foggy conditions.” High beam headlights can glare off of the fog, making visibility even worse for the driver. Parsons’ number two tip is to reduce speed in foggy conditions. It is sometimes necessary to slow down quickly, but Parsons advises tapping the break lights to warn drivers behind of the upcoming fog.
“Drivers can even turn on their emergency flashers to let the people behind them now that they are about to slow down,” Parsons said.
If conditions get too bad, Parsons suggests pulling over and exiting the highway. advises drivers to exit at the nearest off-ramp, or pull over as far as possible on the shoulder, and then turn off their lights.
“If you leave the running lights on somebody driving along in the fog could think you’re in the lane. They could drive right into you, not knowing that you are at a stop,” Parsons said.
To contact Andrea Goodwin, e-mail or call 634-9141 ext. 2003.

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