← CDL General Knowledge: Traffic Laws & Signs

CDL Commercial Drivers License General Knowledge Study Guide

Key concepts, definitions, and exam tips organized by topic.

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CDL General Knowledge: Traffic Laws & Signs Study Guide


Overview

This study guide covers the essential traffic laws and sign recognition knowledge required for the CDL General Knowledge exam. Topics include speed laws, right-of-way rules, traffic control devices, road sign identification, and laws specific to commercial vehicle operation. Mastering these concepts is critical not only for passing the exam but for safe, legal operation of a commercial motor vehicle (CMV).


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Speed Laws & Limits


Key Concepts

Commercial vehicles are held to stricter speed standards than passenger cars due to their size, weight, and longer stopping distances. Two distinct legal concepts govern CMV speed:


  • • Posted Speed Limits — the maximum legal speed under ideal conditions
  • • Basic Speed Law — requires drivers to travel at a speed that is safe for current conditions, even if that means going below the posted limit

    • School Zones

  • • 15 mph is the standard default when children are present and no speed limit is posted
  • • Always reduce speed when approaching schools, regardless of time of day

    • Rural Highways (No Posted Limit)

  • • Federal guidance: drive at a reasonable and prudent speed
  • • Many states default to 55 mph for trucks when no limit is posted

    • Why Commercial Vehicles Have Lower Limits

    | Factor | Impact |

    |---|---|

    | Greater weight | Longer stopping distances |

    | Higher center of gravity | Increased rollover risk |

    | Larger footprint | Reduced maneuverability |


    Key Terms

  • • Basic Speed Law — Never drive faster than is safe for current conditions, regardless of posted limits
  • • Reasonable and Prudent Speed — A speed appropriate for weather, traffic, visibility, and road conditions
  • • Posted Speed Limit — The maximum legal speed under ideal conditions

    • Watch Out For ⚠️

    > The basic speed law can make you legally liable even if you are at or below the posted speed limit. If conditions are poor (rain, fog, heavy traffic), you must slow down further. The exam may present scenarios testing whether you understand this distinction.


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    Right-of-Way Rules


    Key Concepts

    Right-of-way rules determine which driver has the legal priority to proceed. Yielding right-of-way is a legal obligation, not a courtesy option.


    Uncontrolled Intersections

  • • Vehicle that arrives first has the right of way
  • • If arriving simultaneously → vehicle on the right has the right of way

    • Four-Way Stops (Simultaneous Arrival, Vehicles Facing Each Other)

  • • Both going straight or turning right → either may proceed
  • • One turning left → vehicle going straight or turning right has priority
  • • Left-turning vehicles almost always yield

    • Merging onto Highways

  • • Highway traffic always has the right of way
  • • The merging driver must adjust speed to find a safe gap — never force other vehicles to brake

    • Pedestrians

  • • Always yield to pedestrians in marked or unmarked crosswalks
  • • Yield whenever a pedestrian is in your intended path of travel

    • Emergency Vehicles (Lights & Sirens Active)

  • • Immediately pull to the right edge of the road
  • • Come to a complete stop
  • • Remain stopped until the emergency vehicle has fully passed

    • Key Terms

  • • Right of Way — The legal right to proceed before another vehicle or pedestrian
  • • Yield — Slow down or stop to allow another vehicle or pedestrian to proceed first
  • • Uncontrolled Intersection — An intersection with no traffic signs or signals

    • Watch Out For ⚠️

    > Having the right of way does not mean you should proceed blindly. CDL drivers are expected to drive defensively. The exam may test scenarios where yielding your right of way is the safer choice even if you are legally entitled to proceed.


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    Traffic Control Devices


    Traffic Signals


    | Signal | Meaning | Required Action |

    |---|---|---|

    | Steady Red | Stop | Complete stop before stop line/crosswalk; wait for green |

    | Steady Yellow | Caution/prepare to stop | Slow down; stop if safe to do so |

    | Flashing Yellow | Caution | Slow down, proceed with caution; stopping not required |

    | Flashing Red | Treat as stop sign | Full stop, then proceed when safe |

    | Dark/Non-Functioning | Treat as four-way stop | All drivers stop; yield per right-of-way rules |


    Road Markings


    | Marking | Meaning |

    |---|---|

    | Solid White Line (between lanes) | Lane change discouraged or prohibited; stay in lane |

    | Dashed White Line (between lanes) | Lane changes permitted with caution |

    | Double Solid Yellow Center Line | Passing prohibited in both directions; do not cross |

    | Dashed Yellow Line | Passing permitted when safe |


    Key Terms

  • • Traffic Control Device — Any sign, signal, or marking that regulates, warns, or guides traffic
  • • Stop Line — The white line before an intersection where vehicles must stop
  • • Center Line — The line(s) dividing opposing lanes of traffic

    • Watch Out For ⚠️

    > A non-functioning (dark) signal is one of the most commonly missed exam questions. It must be treated as a four-way stop, not as a green light or a yield. Always stop first.


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    Road Signs Recognition


    Sign Shapes & Their Meanings


    | Shape | Type | Example |

    |---|---|---|

    | Octagon | Stop | Stop sign (always red) |

    | Triangle (pointing down) | Yield | Yield sign (red border, white background) |

    | Diamond | Warning | Curve ahead, slippery road |

    | Circle | Railroad advance warning | RR crossing ahead |

    | Pentagon (5-sided) | School zone | School crossing ahead |

    | Rectangle | Regulatory or Guide | Speed limit, route markers |


    Sign Colors & Their Meanings


    | Color | Meaning | Example |

    |---|---|---|

    | Red | Stop, prohibition | Stop sign, do not enter |

    | Yellow | Warning | Curve, hill, signal ahead |

    | Orange | Work/construction zone | Road work, flagger ahead |

    | Green | Guide/directional | Highway distance, exit signs |

    | Blue | Motorist services | Gas, food, lodging, hospital |

    | White | Regulatory | Speed limit, lane control |

    | Brown | Recreation/cultural sites | State parks, historic areas |


    Key Sign Identification Summary


  • • 🔴 Octagonal Red → STOP — complete stop required
  • • 🔺 Triangle, Red Border, White → YIELD — slow and give right of way
  • • 🔶 Diamond, Yellow/Orange → WARNING — hazard ahead
  • • ⭕ Circle, Yellow with RR/X → Railroad crossing — watch for trains
  • • ⬠ Pentagon → School zone — watch for children
  • • 🟩 Green Rectangle → Guide sign — direction and distance
  • • 🔵 Blue Rectangle → Services — gas, food, rest areas

    • Watch Out For ⚠️

    > The exam tests both shape and color recognition independently. Know that a circle always means railroad crossing, a pentagon always means school zone, and an octagon always means stop — regardless of color context. Also, construction/work zone signs are orange diamonds, not yellow.


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    Traffic Laws & Commercial Vehicles


    Move Over Law

  • • When approaching stopped emergency, maintenance, or tow vehicles with flashing lights on the roadside:

    • - Move over one lane away from the stopped vehicle, OR

    - Slow down significantly if a lane change is not possible

  • • Applies in all 50 states; fines and penalties are severe for violations

    • Through Highway & Right of Way

  • • A through highway gives continuous right of way to traffic on the main road
  • • Vehicles entering from side streets or driveways must yield to through traffic

    • Hours of Service (HOS) — Property-Carrying Drivers

  • • Maximum 11 hours of driving after 10 consecutive hours off duty
  • • This is a federal regulation under FMCSA rules

    • No Zone (Blind Spots)

  • • No Zones are the areas around a CMV where the driver has limited or no visibility:

    • - Directly in front of the cab

    - Directly behind the trailer

    - Along both sides (especially the right side)

  • • Other drivers who linger in No Zones are at greatly increased crash risk

    • ```

    [FRONT NO ZONE]

    ↑

    [LEFT] [TRUCK] [RIGHT NO ZONE]

    ↓

    [REAR NO ZONE]

    ```


    Headlight Requirements

    Commercial drivers must use headlights:

  • • At night
  • • In bad weather reducing visibility
  • • In tunnels
  • • In many states, whenever windshield wipers are in use

    • Key Terms

  • • Move Over Law — Legal requirement to change lanes or slow when passing roadside emergency vehicles
  • • Through Highway — A road where traffic has continuous right of way; side street traffic must yield
  • • No Zone — Blind spot areas around a CMV with limited or no driver visibility
  • • Hours of Service (HOS) — Federal regulations limiting driving time to prevent fatigued driving
  • • FMCSA — Federal Motor Carrier Safety Administration; the federal agency that regulates CMV operation

    • Watch Out For ⚠️

    > HOS rules are a common source of exam errors. Remember: 11 hours driving maximum after 10 consecutive hours off for property carriers. Do not confuse with passenger carrier rules (10 hours driving/8 hours off). Also, the Move Over Law applies not just to police cars but to tow trucks and maintenance vehicles as well.


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    Quick Review Checklist ✅


    Use this checklist to confirm you are exam-ready:


  • • [ ] I can explain the basic speed law and how it differs from posted speed limits
  • • [ ] I know the default school zone speed is 15 mph when no limit is posted
  • • [ ] I know that highway traffic has right of way over merging vehicles
  • • [ ] I know at an uncontrolled intersection, first to arrive goes first; ties go to the vehicle on the right
  • • [ ] I know left-turning vehicles must yield to oncoming traffic
  • • [ ] I can identify what to do at a flashing red (stop sign treatment) vs. flashing yellow (caution, slow)
  • • [ ] I know a non-functioning signal = four-way stop
  • • [ ] I can match all sign shapes to their meanings (octagon = stop, triangle = yield, diamond = warning, circle = railroad, pentagon = school)
  • • [ ] I can match all sign colors to their categories (red = stop/prohibition, yellow = warning, orange = construction, green = guide, blue = services, white = regulatory)
  • • [ ] I know the Move Over Law applies to emergency, maintenance, AND tow vehicles
  • • [ ] I know No Zones are blind spots — front, rear, and sides of the truck
  • • [ ] I know property-carrying drivers may drive 11 hours max after 10 consecutive hours off
  • • [ ] I know headlights are required in bad weather and tunnels, not just at night
  • • [ ] I know emergency vehicles with lights/sirens require me to pull right and stop completely

    • ---


    Study Tip: Practice identifying signs by shape first, then color, then text. On the actual CDL exam, sign images may appear without text labels — knowing shapes and colors independently will give you a significant advantage.

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