8.2 Traffic Signal Warrants (MUTCD)

Traffic Signal Warrants (MUTCD)

1. Introduction to Signal Warrants and Engineering Studies

An intersection control signal should not be installed unless an engineering study indicates that the installation will improve the overall safety and/or operation of the intersection. The Manual on Uniform Traffic Control Devices (MUTCD) Chapter 4C defines nine official Traffic Signal Warrants that establish minimum physical and volume conditions under which a traffic control signal should be considered.

CRITICAL DESIGN PRINCIPLE: A signal warrant is not a mandate. Satisfying one or more warrants does not mean a signal must be installed. The decision to install a signal must be supported by professional engineering judgment. Installing a signal at an inappropriate location can increase delay, prompt drivers to use alternative local cut-through routes, and significantly increase rear-end collisions (which often multiply after signalization, though right-angle crashes generally decrease).

2. The 9 Official MUTCD Warrants

Warrant 1: Eight-Hour Vehicular Volume

This warrant is the most commonly utilized and is divided into two primary conditions, which must be satisfied for any 8 hours of an average day:

• Condition A (Minimum Vehicular Volume): Intended for applications where a large volume of intersecting traffic is the principal reason for consideration. It requires high volumes on both the major and minor streets.
• Condition B (Interruption of Continuous Traffic): Intended for applications where the traffic volume on a major street is so heavy that traffic on a minor intersecting street suffers excessive delay or conflict in entering or crossing the major street.
• Combination of A and B: If neither Condition A nor Condition B is fully met, but both are satisfied to at least 80% of their respective volume requirements, Warrant 1 is satisfied.

Warrant 2: Four-Hour Vehicular Volume

This warrant is satisfied if, for any 4 hours of an average day, the plotted points representing the major-street and minor-street volumes fall above the applicable curves in MUTCD Figure 4C-1 (or Figure 4C-2 for high-speed/isolated conditions). It is useful for intersections experiencing high-volume surges during specific periods (e.g., shopping centers, recreational sites).

Warrant 3: Peak Hour

This warrant is intended for locations where traffic conditions for a single hour of an average day are exceptionally heavy. It is satisfied if either of the following is met:

1. The plotted volumes for the peak hour fall above the curves in MUTCD Figure 4C-3.
2. The total delay on one minor-street approach (one direction) is $\ge 4$ vehicle-hours for a one-lane approach, or $\ge 5$ vehicle-hours for a two-lane approach, AND the volume on the minor-street approach is at least 100 veh/h (one lane) or 150 veh/h (two lanes), AND the total entering volume is at least 650 veh/h (for 3-leg intersections) or 800 veh/h (for 4-leg intersections).

Warrant 4: Pedestrian Volume

Intended for locations where high pedestrian volume crossing a major street faces substantial delay. It is satisfied if the plotted pedestrian and vehicular volumes for any 4 hours or any 1 hour fall above the respective MUTCD curves, and there are fewer than 60 clean gaps per hour in the traffic stream.

Warrant 5: School Crossing

Satisfied if an engineering study of the frequency and adequacy of gaps in the traffic stream, relative to the number of school children crossing (minimum of 20 kids), shows that the number of acceptable gaps is less than the number of times children need to cross. It applies specifically to established school routes.

Warrant 6: Coordinated Signal System

Used where signal spacing is such that vehicles would otherwise fail to travel in cohesive groups (platoons). A signal may be justified if it helps maintain proper progression along an arterial and prevents unnecessary stops.

Warrant 7: Crash Experience

This warrant is safety-driven and is met if all of the following conditions are satisfied:

1. Alternative, less restrictive measures (e.g., warning signs, flashers, channelization, enforcement) have been tried and have failed to reduce crash frequency.
2. At least 5 qualifying crashes (right-angle or left-turn collisions susceptible to correction by a signal) have occurred within a 12-month period.
3. The volume warrants (Warrant 1 Condition A, B, or the Combination) are satisfied to at least 80% of their normal requirements.

Warrant 8: Roadway Network

Justified to encourage concentration of traffic on a major roadway network. It requires an entering volume of at least 1,000 veh/h during the peak hour of an average weekday and meeting specific system connectivity criteria.

Warrant 9: Grade Crossing

Applies to intersections close to a highway-rail grade crossing. It is satisfied if the peak hour volume and distance from the stop line to the tracks indicate a severe risk of traffic queuing back onto the tracks.

3. Standard MUTCD Volume Thresholds (Warrant 1)

Below are the standard 100% volume thresholds for Warrant 1. These represent the minimum hourly volumes (veh/h) required on the major street (total of both approaches) and the higher-volume minor-street approach (one direction only) for 8 hours of an average day.

4. Special Factors: The 70% Reduction Factor

MUTCD allows a 70% reduction in the volume thresholds of Warrant 1 (Conditions A and B), Warrant 2, and Warrant 3 if either of the following conditions is met:

1. The design speed or 85th-percentile speed on the major street exceeds 40 mph (70 km/h).
2. The intersection is located within the built-up area of an isolated community with a population of less than 10,000.

Under this factor, the volume thresholds are multiplied by 0.70 (e.g., for a 1-lane/1-lane intersection under Condition A, the major street volume threshold drops from 500 to 350 veh/h, and the minor street from 150 to 105 veh/h).

5. Practical PE Exam Design Tips

• Read the Speed Limit: Look for the approach speed. If it is 45 mph or higher, immediately apply the 70% volume reduction factor to Warrants 1, 2, and 3.
• Crash Experience Trap: The 5 crashes in 12 months must be susceptible to correction by a signal. Rear-end collisions and single-vehicle run-off-road crashes are typically NOT correctable by a signal (in fact, rear-end crashes often increase). Do not count them toward the 5-crash threshold unless the problem explicitly tells you they are qualifying.
• Minor Street Volume: When checking the tables, the minor street volume is always for the higher-volume minor approach in a single direction, NOT the sum of both minor approaches. The major street volume, however, is the total of both approaches.