Traffic Engineering

Capacity and level of service, the traffic studies and forecasts that feed them, and crash-based safety analysis. At 10–15 of 80 it is the largest area on the Transportation exam, with eight subtopics running from freeway LOS to the Highway Safety Manual.

What the problems look like

Mostly calculation, with table and chart reads. Recurring shapes — freeway and multilane density and LOS; urban street travel speed; signalized saturation flow, v/c, critical lane groups and approach delay; two-way stop and roundabout capacity; PHF and speed studies; count factors, AADT and VMT; lanes required for a design year; the gravity model; pedestrian and bicycle LOS; and the HSM — crash rates, EPDO, SPFs with CMFs, empirical Bayes, before/after evaluation.

By exam day you should be able to —

• turn a count into a peak 15-minute flow rate in pc/h/ln;
• carry a facility through the HCM steps to density, speed, delay or v/c, then to LOS;
• adjust short counts to AADT and distribute trips with the gravity model;
• compute crash rates per MEV (intersections) or per million VMT (segments), rank sites by severity, and screen with the critical rate;
• predict crashes with an SPF × CMFs × calibration factor and blend in history with empirical Bayes.

Where people lose points

• PHF: the peak hour is the busiest four consecutive intervals, not the clock hour; V15 comes from inside that hour; and the hourly volume is divided by the PHF.
• Speeds: space-mean speed is total distance over total time; averaging individual speeds gives time-mean speed.
• Stated conditions: when a measured free-flow speed is given, use it to enter the speed–flow relation. The posted limit comes in only where the method calls for it. Treat a long, steep grade as a specific grade, not as general terrain.
• Signals: a lane group's flow ratio includes its lane count; criticality is decided by flow ratio, not volume; approach delay is flow-weighted across lane groups.
• Urban street travel time adds only the downstream boundary signal's delay, and each direction is rated on its own.
• HSM: CMFs multiply, each applying only to its own setting, crash type and severity; empirical-Bayes weights sum the predicted crashes over the study years; with whole-calendar-year study periods, the installation year is normally left out of both the before and the after period.
• Counts and exposure: with day and month factors defined as in the Handbook (average over count), they multiply the count. When one weekday figure represents all weekdays, weight it five times in the weekly average. Build MEV and VMT over the full study period and on each segment's own length.

Have these at your fingertips

The HCM's facility chapters, HSM Parts B, C and D, and the traffic-study relations in the Handbook. The HCM edition you are supplied depends on your appointment date — the 6th edition before the April 2027 exam, the 7th from it — so work from the edition listed for your exam window.

Going deeper

• Transportation design standards: the April 2027 HCM and MUTCD change
• Traffic Signals — timing the intersections whose capacity you rate here

Part of the Transportation exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.