Driving Safely
The core idea
Safe driving on the CDL General Knowledge test is mostly about space, speed, and sight distance. You must know how to judge following distance, how to scan ahead, and how to react when conditions change.
Concepts to master
- Following distance: The minimum for a CMV under 40 feet is one second for each 10 feet of vehicle length at speeds under 40 mph; add one second above 40 mph. A 60-foot vehicle at 40 mph needs 6 seconds; at 55 mph it needs 7.
- Total stopping distance = perception distance + reaction distance + brake lag + braking distance.
- Scanning: Look 12-15 seconds ahead in cities, about a quarter mile on highways.
- Hazard perception: Watch for drivers entering from side streets, brake lights, and stale green lights.
- Distracted and drowsy driving: Both are treated as serious hazards; fatigue impairs judgment before you feel sleepy.
Traps
Questions love to swap "perception distance" with "reaction distance," or ask what to do when a tire blows (hold the wheel firmly, stay off the brakes, ease off the throttle). Another common trap: hydroplaning begins at speeds as low as 30 mph with low tread.
Worked example
A 70-foot combination travels at 50 mph. Following distance = 7 seconds (one per 10 feet) plus 1 second for speed over 40 = 8 seconds. At 50 mph a vehicle covers about 73 feet per second, so 8 seconds is roughly 580 feet - about two city blocks.
Rule of thumb
If you cannot see the rear tires of the vehicle ahead touching the road, you are too close.