CDL General Knowledge Test · CDL General Knowledge Test
CDL General Knowledge Test: Seeing and Communicating Concepts
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Overview
This chapter covers how to see what is happening around a large vehicle and how to let other road users know what you are doing. Not looking properly is a major cause of crashes, and other drivers may not notice even a big truck in plain sight.
Look 12 to 15 seconds ahead, which is about one block in town and about a quarter mile on the highway, and shift your eyes between near and far. Check your mirrors often, and more often during lane changes, turns, merges, and tight moves. Convex mirrors show a wider view, but they make things look smaller and farther away than they are.
Signal early and keep signaling until the turn is done. Flash your brake lights to warn drivers behind you when you will slow down, and do not wave other drivers by. When you stop on a road or shoulder, turn on your flashers and put out warning devices within 10 minutes. On a two-way road, place them within 10 feet of the front or rear corner, 100 feet behind, and 100 feet ahead. On a one-way or divided highway, place them 10, 100, and 200 feet back toward traffic.
Key concepts
Seeing ahead
Look 12 to 15 seconds ahead of your vehicle. In town that is about one block, and at highway speed it is about a quarter of a mile.
Memory tip12 to 15 seconds: a block in town, a quarter mile on the highway.
Source: New York State DMV Commercial Driver's Manual (CDL-10), Section 2 · as of 2026
Convex mirror
A curved mirror that shows a wider area than a flat mirror. Everything in it looks smaller and farther away than it really is.
Memory tipCloser than it looks.
Warning devices: time limit
When a commercial vehicle stops on the traveled part or shoulder of a highway (other than for traffic), the driver must put out warning devices as soon as possible and within 10 minutes.
Memory tipFlashers on at once, triangles out within 10 minutes.
Source: 49 CFR 392.22 (FMCSA) · as of 2026
Triangles on a two-way road
On a two-lane road with traffic both ways, or an undivided highway: one device within 10 feet of the front or rear corner, one 100 feet behind, and one 100 feet ahead.
Memory tipTwo-way traffic, so warn both ways: 10, 100 behind, 100 ahead.
Source: 49 CFR 392.22 (FMCSA) · as of 2026
Triangles on a divided highway
On a one-way or divided highway: place devices 10 feet, 100 feet, and 200 feet back toward the approaching traffic.
Memory tipOne-way traffic, so all three go behind: 10, 100, 200.
Source: 49 CFR 392.22 (FMCSA) · as of 2026
Do not direct traffic
Do not signal other drivers that it is safe to pass. You could cause a crash, and you could be blamed for it.
Memory tipDrive your own vehicle, not theirs.
Often tested
- Look 12 to 15 seconds ahead.
- Convex mirrors make things look smaller and farther away.
- Flash your brake lights to warn drivers behind you before slowing.
- Put out warning devices within 10 minutes of stopping.
- Two-way road: 10 feet, 100 feet behind, 100 feet ahead. Divided highway: 10, 100, and 200 feet behind.
Easy to confuse: Where to place warning triangles
| Road type | Placement |
|---|---|
| Two-lane road, traffic both ways | Within 10 feet of the front or rear corner, 100 feet behind, 100 feet ahead |
| One-way or divided highway | 10 feet, 100 feet, and 200 feet back toward approaching traffic |
3 sample questions
Verified practice questions from this unit.
Question 1. How far ahead should you look while driving?
- ① 4 to 5 seconds ahead
- ② 30 to 40 seconds ahead
- ③ 12 to 15 seconds ahead
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Answer: ③ 12 to 15 seconds ahead
Key point: Look 12 to 15 seconds ahead
Good drivers look 12 to 15 seconds ahead. In town that is about one block. On the highway it is about a quarter of a mile. Keep shifting your eyes between near and far.
Wrong choices
- 4 to 5 seconds ahead: This is not far enough. You may have to stop or change lanes suddenly.
- 30 to 40 seconds ahead: This is farther than the rule. You also need to watch closer things.
Question 2. How do objects look in a convex (curved) mirror?
- ① Smaller and farther away than they really are
- ② Larger and closer than they really are
- ③ Exactly the same as in a flat mirror
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Answer: ① Smaller and farther away than they really are
Key point: Convex mirror: smaller and farther away
Convex mirrors show a wider area than flat mirrors. But everything looks smaller and farther away than it really is. Allow for this when you judge space.
Wrong choices
- Larger and closer than they really are: This is the opposite of what a convex mirror does.
- Exactly the same as in a flat mirror: Convex mirrors show a wider area but change how things look.
Question 3. You must stop on the shoulder of a highway. How soon must you put out your warning devices?
- ① Within 30 minutes
- ② Within 10 minutes
- ③ Only after dark
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Answer: ② Within 10 minutes
Key point: Put out warning devices within 10 minutes
When you stop on the road or shoulder, turn on your four-way flashers and put out your warning devices within 10 minutes, day or night.
Wrong choices
- Within 30 minutes: 30 minutes is too long. Federal rules require 10 minutes.
- Only after dark: You must put them out day or night.