Slow down in the fog

With the approaching fog season it would be appropriate to cover some basics about driving in fog. In a previous column we covered what is known as perception and reaction time. When a person drives in the fog perception and reaction time is crucial to personal safety and the safety of others.

To review perception and reaction time I will cover some basic concepts. The National Highway Traffic Safety Administration (NHTSA) states the perception and reaction time for average driver is 1.5 second, .75 seconds to perceive a threat in the roadway and .75 seconds to react to the threat in the roadway. This perception and reaction time directly relates to driving in the fog because visibility is greatly affected. The example I use is to imagine driving down the road, if a vehicle was disabled in the middle of the road or a person was crossing the road would I be able to safely stop before a collision occurred. Fog is the best example of people violating the basic speed law.

Vehicle Code Section 22350 states, “No person shall drive a vehicle upon a highway at a speed greater than is reasonable or prudent having due regard for weather, visibility, the traffic on, and the surface and width of, the highway, and in no event at a speed which endangers the safety of persons or property.” This law is also known as the basic speed law.

After viewing several studies on the affects of driving in the fog, a majority of drivers do not show a tendency to drive faster in the fog but the studies do show that drivers tend to overestimate how far the drivers can clearly see. This is an example of how fog affects speed on the freeway. The posted maximum speed limit on Highway 113 is 65 MPH. When visibility is decreased to 300 feet the safe speed would be approximately 57 MPH. As visibility decreased so does the safe speed. To help drivers determine how fast they should drive when traveling in low visibility conditions, here list of speeds appropriate for conditions. 300 feet visibility is 57 MPH, 200 feet is 44 MPH, 100 feet is 27 MPH, and 50 feet is 16 MPH. The speeds are suggestions to safe speeds based on stopping distance calculations. It is up to the drivers to determine what a safe speed for the conditions would be. In a previous column we discussed the different law that is used to enforce maximum speed limits but the basic speed law applies in all circumstances.
This week I received an e-mail question about placement of stop limit lines at intersections. For the answer I spoke with City of Woodland Traffic Engineer Shawn Fisher. The limit lines in Woodland are generally placed where a crosswalk is or would be placed. Where a marked crosswalk is present, the first crosswalk line is where vehicles should stop. One situation that affects the placement of crosswalks is the location of wheelchair access ramps. Per the code book used by traffic engineers (MUTCD), a crosswalk or limit line is not allowed prior to a wheelchair access ramp.

The intersection that receives the most inquiries is the intersection of Hays Ln. and West St. At the intersection the limit line on Hays Ln. is placed back from the intersection approximately 20 feet. This placement is caused by a conflict with the wheelchair access ramp and a storm drain in the street.

Thank you for reading the article and you are welcome to submit questions through the www.woodlandpolice.org