CDL: Safe Speeds for Different Road Surfaces
Matching Speed to the Road Surface
Speed limits are set with normal, dry conditions in mind — but the posted number isn't always a safe number. A commercial driver's real job is to judge how much traction the road is actually offering at any given moment, and adjust speed accordingly. This article covers how to recognize slippery surfaces before they catch you off guard, and what to do if your vehicle starts to hydroplane.
Traction Is Everything
Steering and braking are only possible when your tires have traction — the friction between your tires and the road surface. Certain road conditions reduce that friction significantly, and recognizing them early is what allows you to slow down before it becomes a problem rather than after.
Slippery Surfaces
When the road is slippery, stopping distances grow longer and turning without skidding becomes harder. A wet road can double your normal stopping distance, which means you need to drive slower to be able to stop within the same distance you'd need on dry pavement. As a general guideline:
- On a wet road, cut your speed by roughly a third — for example, slow from 55 mph down to about 35 mph.
- On packed snow, reduce your speed by half or more.
- On an icy surface, slow to a crawl, and stop driving altogether as soon as you can safely do so.
Recognizing a Slippery Surface
Slippery conditions aren't always obvious. Here are some clues that can tip you off:
- Shaded areas: Sections of road in shade stay icy and slick long after sunny stretches have already thawed.
- Bridges: As temperatures fall, bridges freeze before the surrounding roadway does. Be especially cautious anytime the temperature is hovering near 32 degrees Fahrenheit.
- Melting ice: Ice that's just starting to melt becomes wet, and wet ice is considerably more slippery than ice that's dry.
- Black ice: This is a thin, nearly invisible layer of ice that lets you see the pavement underneath, making the road look merely wet rather than icy. Whenever the temperature is below freezing and the road appears wet, treat it as a potential black ice situation.
- Vehicle icing: A simple way to check for ice buildup is to roll down your window and feel the front of your mirror, its support arm, or your antenna. If ice has formed there, the road surface is likely starting to ice up as well.
- Just after rain begins: When rain first starts falling, it mixes with oil residue left on the road by other vehicles, creating an especially slick surface. As the rain continues, it gradually washes that oil away, improving traction.
Hydroplaning
In certain weather conditions, water or slush can build up on the roadway to the point where your vehicle begins to hydroplane. It works much like water skiing: your tires lose contact with the road surface and provide little or no traction, which can leave you unable to steer or brake effectively.
If you find yourself hydroplaning, ease off the accelerator and push in the clutch. This slows the vehicle down and allows the wheels to turn freely again. Do not apply the brakes while hydroplaning. If the drive wheels start to skid, pushing in the clutch lets them rotate freely rather than fighting against the skid.
Standing water isn't always easy to spot from the driver's seat, but there are visual clues worth watching for: clear reflections on the pavement, splashes kicked up by tires ahead of you, and raindrops bouncing visibly off the road surface. All of these point to water pooling on the roadway — exactly the kind of condition that can lead to hydroplaning.
Matching your speed to the road surface isn't about hitting an exact formula every time — it's about staying alert to the clues around you and being willing to slow down well before a slippery patch turns into a loss of control.