Intersection Geometry

Where roads meet: intersection sight distance, interchange ramps and terminals, and at-grade layout — turn lanes, islands, turning roadways and roundabouts. It carries 7–11 of the 80 questions on the Transportation exam.

What the problems look like

Plan-view calculation, Green Book lookups and some design-judgment items. Recurring shapes — a sight-triangle leg or the setback an obstruction needs; acceleration and deceleration lanes, level or on grade; lane balance and ramp spacing; ramp design speed; left-turn lane length with storage; turning-roadway width for a truck mix; island size; and roundabout and channelization principles.

By exam day you should be able to —

• name the ISD case from the control and permitted movements, and compute the leg as speed × gap;
• adjust the gap for lanes crossed, median, design vehicle and grade;
• solve a setback from the sight triangle by similar triangles;
• size speed-change lanes from the highway speed and the ramp's controlling feature, then correct for grade;
• size a turn lane — taper, deceleration, storage — and a turning roadway for its governing vehicle.

Where people lose points

• Read the control first: a NO LEFT TURN sign or a right-only channelized exit changes the case, the gap and which legs exist. Stop and yield control use different gaps.
• Gap adjustments count lanes beyond a base that differs by maneuver. A median that can't store the design vehicle with end clearance becomes equivalent lanes. Trucks take a larger increment, and only a minor-road approach upgrade steeper than 3 percent adds time.
• Triangle geometry: the vertex is set back from the edge of traveled way, and traffic from the left runs in the near lane. The major-road leg is intersection sight distance, not stopping sight distance.
• Speed-change lanes: enter with the design speed of the controlling feature — the ramp curve, or a stop — not an advisory speed. Judge upgrade or downgrade in the direction of travel, and apply a grade factor only outside the flat-grade band.
• Lane balance sets a floor on lanes beyond a merge, not a fixed count.
• Ramp spacing: classify the pair — successive entrances or exits, exit then entrance, or a weaving entrance then exit — and system or service, full freeway or C-D road, before reading a value.
• Storage and turning roadways: on a protected-only left phase, size storage from arrivals per cycle, not gap acceptance. Enter a turning-roadway width table with the inner-edge radius.

Have these at your fingertips

ISD = 1.47 × V × t_g, similar-triangle setbacks, and storage from arrivals per cycle. These come from the Green Book, not the Handbook; know where the Handbook's signal-timing and curve relations live. Design values come from the Green Book (7th ed., 2018) — Chapter 9 for intersections, Chapter 10 for interchanges, Chapter 2 for design vehicles and Chapter 3 for turning-roadway widths — supplied for both the current and the April 2027 exam windows.

Going deeper

• Traffic Signals — the cycle that sets left-turn storage
• Horizontal Design — the curve relations behind ramp speed

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.