Horizontal Design

The road in plan: circular-curve elements and stationing, sight distance on the inside of a curve, superelevation and its transitions, and the special cases — compound, reverse and spiral curves, widening, and coordination with the profile. It carries 8–12 of the 80 questions on the Transportation exam.

What the problems look like

Mostly calculation from a plan figure, plus some Green Book judgment items. Recurring shapes — PC and PT stations from a PI (sometimes one found by traverse); degree of curvature; stakeout deflections; the sight line offset to a building or cut face; minimum radius and design superelevation; runoff and runout placement; traveled-way widening for a truck; and compound and reverse curve geometry.

By exam day you should be able to —

• work T, L, E, M and the long chord from R and Δ, and station a curve from its PI;
• convert R and D under the arc or chord definition;
• find the sight line offset a curve needs, or the design speed an obstruction allows;
• size runoff and runout for the lanes rotated, and locate them on the tangent and curve;
• solve compound and reverse curves, and compute widening from the vehicle's wheelbases.

Where people lose points

• Stationing follows the arc: PC = PI − T and PT = PC + L. PI + T is not the PT, and the long chord is not the curve length. Deflections are measured from the PC.
• Degree of curvature depends on the radius alone; curve length is PT minus PC, not a tangent length.
• Sight line offset runs from the centerline of the inside lane, not the highway centerline or the pavement edge, and holds when the curve is longer than the sight distance. Where the grade adjustment applies, use the stopping distance for the inside lane's direction of travel.
• Supporting a design speed: a speed qualifies only if its stopping sight distance fits within what is available. Not the nearest row, and not braking distance alone.
• Runoff uses the design rate, not e_max, and the design speed, not the posted speed. Count the lanes actually rotated. Where the road returns to normal crown between reverse curves, the tangent carries each curve's tangent share of runoff plus its own runout.
• Widening and spirals: offtracking uses all the wheelbases. A tabulated widening value is for one design vehicle — adjust it for yours. The minimum spiral is the larger of its two criteria.

Have these at your fingertips

Circular-curve, compound- and reverse-curve relations and the law of sines, and where they live in the Handbook. The simplified curve equation R = V²/[15(0.01e + f)], the sight line offset relation and the spiral and offtracking relations come from Green Book (7th ed., 2018) Chapter 3, supplied for both the current and the April 2027 exam windows; so do the design values. Know which Chapter 3 table answers which question.

Going deeper

• Vertical Design — the profile this alignment is coordinated with

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.