General Knowledge: Extreme Conditions

2 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 392.14, Heat transfer: a bridge deck loses heat from both faces

What you need to know

392.14 pairs extreme caution with a reduction in speed, and the list covers traction as well as visibility.

If it gets dangerous enough the rule stops you driving at all.

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.

49 CFR 392.14

A road sits on ground that holds heat and feeds it up into the surface.

A bridge deck has air underneath instead, so it loses heat from both faces and reaches freezing first, often while the approach road is still wet.

*Why:* A pavement on grade loses heat from one face and gains conducted heat from the subgrade, whose thermal mass holds it near its long-run temperature. A bridge deck is bounded by air on both faces, so it has two convective loss paths and no conductive gain. Its surface temperature therefore falls faster and crosses 0 C earlier than adjacent pavement under the same conditions.

Check yourself

• What does the rule require of you when ice affects traction?

• Why does a bridge deck ice over before the road either side of it?

Answers are in the practice test for this topic.