General Knowledge: Extreme Driving Conditions
8 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.
Sources: 49 CFR 392.14, TC 21-305-20 para 6-32, TC 21-305-20 para 6-42, TC 21-305-20 para 6-43
What you need to know
392.14 requires extreme caution when hazardous conditions adversely affect visibility or traction.
This is a higher standard than ordinary or reasonable care.
Extreme caution in the operation of a commercial motor vehicle shall be exercised when hazardous conditions, such as those caused by snow, ice, sleet, fog, mist, rain, dust, or smoke, adversely affect visibility or traction. Speed shall be reduced when such conditions exist. If conditions become sufficiently dangerous, the operation of the commercial motor vehicle shall be discontinued and shall not be resumed until the commercial motor vehicle can be safely operated.
49 CFR 392.14
392.14 contains a passenger-safety exception: where compliance would increase hazard to passengers, the vehicle may be operated to the nearest point at which passenger safety is assured.
It is not a license to complete the route. You may operate only that far.
Whenever compliance with the foregoing provisions of this rule increases hazard to passengers, the commercial motor vehicle may be operated to the nearest point at which the safety of passengers is assured.
49 CFR 392.14
Double it.
If you normally hold two seconds, hold four on wet pavement. The manual also notes that the first few drops of rain are the danger signal, because that is when oil and dust lift off the surface into a slippery film. A four-second rule in place of a two-second rule.
On wet pavements, allow at least two times the normal following distance. For example, when following on wet pavement, use a four-second rule instead of a two-second rule.
TC 21-305-20 para 6-32
Light rain lifts accumulated oil and dust off the surface without washing it away, leaving a slippery film.
Heavy sustained rain actually rinses it off. So the most dangerous moment is the start of the rain, not the middle of it. That calls for slowing down and extra caution, exactly when most drivers have not reacted yet.
To the safe operator, the first few drops of rain are danger signals telling him/her to slow down and be extra cautious.
TC 21-305-20 para 6-32
The first few drops lift accumulated grease and dirt into a film without washing it away.
Later rain dilutes and clears it, so the most dangerous moment is the one that feels least serious.
many do not realize that roads are likely to be especially slick just after rain or drizzle begins. The first few drops loosen grease and dirt accumulated on the road surface
TC 21-305-20 para 6-32
A danger signal.
Most drivers respond to rain when it becomes heavy enough to hurt visibility, which is already past the slipperiest point. The first drops lift grease and dirt without washing them away. The road is about to be at its most slippery.
To the safe operator, the first few drops of rain are danger signals
TC 21-305-20 para 6-32
The surface moves under the tire and the curves give you no bank to help.
Both lengthen stopping distance and make skids and spinouts likelier at every speed.
Gravel roads are particularly dangerous because of their loose surface and flat (unbanked) curves. Stopping distances at all speeds are greater and skids and spinouts are more likely to occur.
TC 21-305-20 para 6-43
40 to 45 mph, regardless of what is posted.
A posted limit is a maximum for good conditions, not a statement that the road supports it.
Probably the highest reasonable speed on most of them is 40 MPH to 45 MPH.
TC 21-305-20 para 6-42
Check yourself
- What is the standard of care that applies when snow or ice affects traction?
- What is allowed a bus driver when stopping would endanger passengers?
- How much following distance does the manual call for on wet pavement?
- What is the significance of the first few drops of rain?
- When is a road most slippery in a rain shower?
- What should the first few drops of rain on your windshield tell you?
- What two things make gravel roads particularly dangerous?
- What does the manual give as the highest reasonable speed on most secondary roads?
Answers are in the practice test for this topic.