Roundabout capacity on the PE Civil Transportation exam

A roundabout capacity problem gives you turning-movement flows and a lane configuration, then asks for one entry's capacity or its volume-to-capacity ratio. The arithmetic is one exponential. Most lost points come earlier, in deciding which flows conflict with the entry and which equation fits it. The capacity method is in the Highway Capacity Manual's roundabouts chapter. The design side (splitter islands, entry speed, bypass lanes, multilane path overlap) is in the Green Book's sections on roundabouts in Chapter 9. The Handbook has no roundabout relations, so both come from the supplied standards.

What you actually need to own

• Circulating flow. The flow that conflicts with an entry is everything passing in front of its splitter island. That's the vehicles that entered upstream and haven't exited yet: the next leg upstream's throughs, lefts and U-turns, the second leg's lefts and U-turns, and the third leg's U-turns. Work it in passenger car equivalents and draw the circulation (counterclockwise) before you add anything.
• Which equation. The HCM gives entry capacity as an exponential decay in circulating flow, c = A·e^(−B·v_c), with separate coefficients for each combination of entry lanes and conflicting circulating lanes, and separate left- and right-lane equations for a two-lane entry. Count both lane numbers from the drawing, then pick. With two circulating lanes, v_c is the total of both.
• Lane assignment on a two-lane entry. Before you split the flow between lanes, check whether one lane works as a de facto exclusive lane. For an entry marked left–through / through–right: if the U-turns plus lefts outweigh the throughs plus rights, the left lane is effectively a left-turn lane. Assign flows that way, compute each lane's v/c, and report the higher one.
• Exiting flow and bypass lanes. Exiting flow enters the capacity check only for a right-turn bypass lane. With no bypass lane, it's information you don't use.
• Pedestrians. The HCM reduces entry capacity for pedestrians crossing the entry. On a one-lane entry the adjustment fades as circulating flow rises; in one practice problem it's already 1.0 at 900 pc/h circulating.
• The design screen. The Green Book's planning check sums the entering and conflicting volumes at an entry: a single-lane entry is likely to be sufficient below about 1,000 veh/h and may be sufficient at about 1,300. That screens the lane count. The Green Book itself calls for a detailed capacity evaluation to confirm it, which is the HCM calculation.

Where people lose points

• Putting the subject leg's own movements into v_c. Its left turns enter at that leg. They don't pass in front of it. In one problem, adding them drops a 642-pc/h capacity to 545.
• Counting right turns that have already exited. A right turn from the upstream leg leaves before it reaches the subject entry.
• Leaving out U-turns. A U-turn from any other leg passes in front of this entry.
• Swapping the lane counts. A one-lane entry against two circulating lanes isn't the same equation as a two-lane entry against one. Picking the wrong one, or the single-lane equation, is a ready distractor.
• Splitting a two-lane entry 50/50. Do the de facto lane check first. The critical lane can be the left one carrying U-turns and lefts.
• Treating geometry as free. Per the Green Book, widening the entry or enlarging the entry radius for a design truck can raise entering speeds. Right-turn bypass lanes add capacity but should be discouraged where pedestrians and bicyclists are active. Lanes that the early years don't use may raise crashes, which is why a phased, single-lane opening is worth considering.

A note on standards

The exam supplies the HCM 6th edition through March 2027 and the 7th edition from April 2027. Work from the edition listed for your exam window. Our HCM-based problems are set on the 7th edition and labelled for the April 2027 window. The Green Book (7th edition, 2018) applies in both windows. See the April 2027 HCM and MUTCD change.

How to study it

Draw a four-leg roundabout and fill in a full turning-movement table. For each entry, list by hand which movements pass in front of it, then check the list by tracing paths around the circle. Run one entry as single-lane, then as one entry lane against two circulating lanes, then as a two-lane entry with the de facto check. The same capacity-and-v/c thinking carries into freeway and multilane LOS and traffic signal warrants.

Part of the Traffic Engineering area of the PE Civil Transportation exam. → Start practicing free — the whole practice bank, free with an account.