Speed guns in the fog?

[chippie_dee]

Can a radar gun work in semi-thick fog? Not that I would speed or anything...

[Junkie]

the range may be decreased somewhat, but it should work fine.

[motorman4life]

Both radar and LIDAR are adversely affected by rain, drizzle and fog. That is not to say they don't work in bad conditions, just that the range (distance) is reduced. It is thought that the worse conditions are, the less reliable the device is. I have conducted field testing that actually contradicts that. If you can see it, you can generally get a reflected signal.

In cases where the fog is so heavy that you cannot see more than 5, 10, 50 feet, than obviously you cannot see vehicles beyond that distance and you cannot get the requisite visual estimate necessary to adequately determine the speed, exclusive of the radar. But, with radar, I have actually gotten accurate readings of vehicles in heavy fog. Because the radar can penetrate the fog, you can actually get a reflected signal in cases where you cannot even see the approaching traffic. On the other hand, I have been unable to get LIDAR to work in such heavy fog.

[Junkie]

can you give an unsafe for conditions ticket without a speed estimate?

[saizai]

IANAC but my guess is that if your line of sight is 50 feet then you're not allowed to go faster than you could completely avoid a major obstacle [e.g. flipped over big rig] in 50 feet worth of time. (AKA "don't go faster than you can see".)

I'm too lazy to do that calculation right now but perhaps MM4L knows it. :P

... eeeenh, or maybe I will.

Usual rule of thumb for bikes IIRC is you should be able to see (speed in mph)/10 seconds ahead of you, +/- 1 depending on skill.

@ 50mph, you cover 50 feet in 0.7s, which is waaay less than necessary. (Absolute minimum reaction time is ~.5s.)

@ 18 mph, you cover 50 feet in ~1.9s, which is barely adequate.

(Go to Google and type e.g. "50 feet / 18 mph" and you'll get the numbers easily.)

FWIW, 65 mph = 620 feet (=0.11 mi) in 6.5s. 85 mph = 1060 feet (=0.2 mi) in 8.5s. So that is probably the cutoff for visibility limiting speed. Assuming all other factors are normal... which they probably aren't if your visibility is that low (think: "very slick roads and traffic in other lanes").

[NorCalBusa]

Same Lane mode is terrific for running down Hwy 5 in heavy fog- it paints the traffic ahead LONG before I can see it.

[Nick]

It's good to always follow the rule of 3 seconds. A 3 second gap in between you and the car in front, 3 second gap in between you and view horizon if the conditions are poor, etc...