CDL: Adjusting Speed for Curves, Traffic, and Downgrades
Controlling Speed: Curves, Visibility, Traffic Flow, Downgrades, and Work Zones
Picking the right speed isn't just about staying under the posted limit — it's about matching your speed to what the road, the traffic, and your vehicle can actually handle at any given moment. This article covers five specific situations where speed decisions carry extra weight: curves, limited visibility, heavy traffic, downgrades, and work zones.
Speed and Curves
Every curve in the road requires a speed adjustment. Take a curve too fast, and one of two things is likely to happen: the tires lose their grip and the vehicle continues in a straight line, sliding off the road, or the tires hold their grip and the vehicle rolls over instead. Trucks with a high center of gravity are especially vulnerable — testing has shown that some can roll over even while traveling at the posted speed limit for a curve.
The safest approach is to slow down before you ever reach the curve. Braking while already in the middle of a turn is risky, since it's much easier to lock the wheels and trigger a skid under those conditions. Never exceed the posted speed limit for a curve, and try to be in a gear that allows for a slight amount of acceleration through the turn — this helps you maintain better control of the vehicle as you come through it.
Speed and How Far You Can See
A good rule of thumb: you should always be able to stop within the distance you can actually see ahead of you. Conditions like fog or rain can shrink that visible distance significantly, which means slowing down enough to still be able to stop safely within whatever range you can see.
The same principle applies at night. Low-beam headlights simply don't illuminate as far down the road as high beams do, so whenever you're relying on low beams, that's a signal to reduce your speed accordingly.
Speed and the Flow of Traffic
When traffic is heavy, the safest speed to drive is usually the same speed as the vehicles around you. Vehicles traveling in the same direction at matching speeds are far less likely to collide with one another. Keep in mind that in many states, posted speed limits for trucks and buses run lower than the limits for passenger cars — sometimes by as much as 15 mph — so extra caution is warranted whenever you're changing lanes or passing on these roads. As a general practice, match the flow of traffic as long as doing so stays legal and safe, and always maintain a safe following distance.
Many drivers who exceed the speed limit do so hoping to save time, but the reality rarely works out that way. Driving faster than the surrounding traffic usually just means repeatedly passing other vehicles, which raises your crash risk and adds to driver fatigue — and fatigue itself increases the chances of a crash. Moving with the flow of traffic, rather than fighting against it, tends to be both safer and considerably less tiring.
Speed on Downgrades
Gravity takes over on a downgrade, naturally increasing your vehicle's speed as you descend. Your job is to select and hold a speed that accounts for several factors at once:
- The total weight of the vehicle and its cargo
- The length of the grade
- How steep the grade is
- Current road conditions
- Weather conditions
If a speed limit is posted for the grade, or if you see a "Maximum Safe Speed" sign, never exceed that number. Also watch for and follow any warning signs indicating how long or steep the upcoming grade is.
Your engine's braking effect should be your primary way of controlling speed on a downgrade — and that braking effect is strongest when the engine is near its governed rpm range and the transmission is in a lower gear. Save your actual brakes so they're available when you need to slow or stop for road and traffic conditions. Shift into a low gear before you start down the grade, and apply proper braking technique throughout the descent.
Speed in Roadway Work Zones
Speeding is the single leading cause of injuries and deaths in roadway work zones. Because of that, it's essential to follow posted speed limits both as you approach a work zone and while driving through it. Keep a close eye on your speedometer, since it's easy for speed to creep back up gradually over the course of a long stretch of construction. Slow down further whenever weather or road conditions call for it, and slow down even more when a worker is close to the roadway.
Whether you're navigating a curve, driving through fog, keeping pace with traffic, descending a long grade, or passing through a work zone, the underlying principle is the same: your speed needs to match the demands of the situation in front of you, not just the number posted on the nearest sign.