Roundabout Versus Signalized Intersection Under High Pedestrian Demand
A suburban intersection of two arterials (one 4-lane, one 2-lane) is being evaluated for conversion to a roundabout. Peak-hour volumes are 1,800 veh/h on the major approach and 900 veh/h on the minor approach. Pedestrian demand is high: 250 pedestrians per hour cross the major approach during the peak, including school children. The existing signalized intersection has a 90 s cycle and pedestrian signals. The roundabout design proposes a single-lane circulating roadway with splitter islands and crosswalks set back from the yield line. The engineer must decide whether the roundabout is appropriate given the pedestrian demand.
1. Evaluate the roundabout's capacity and pedestrian safety implications given the volumes and pedestrian demand.
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Capacity: a single-lane roundabout has a capacity of roughly 1,200–1,500 veh/h per approach depending on the circulating flow. With 1,800 veh/h on the major approach, a single-lane roundabout is likely over capacity - the major approach alone exceeds the typical single-lane capacity. A two-lane roundabout would be needed, but the problem describes a single-lane circulating roadway.
Pedestrian safety: roundabouts are generally safer for pedestrians than signals when crosswalks are set back and splitter islands are provided, because pedestrians cross one direction of traffic at a time and vehicle speeds are lower. However, with 250 pedestrians per hour (including school children), the crossing must be designed carefully:
- Crosswalks should be set back at least one vehicle length from the yield line so drivers can stop without blocking the circulating lane.
- Splitter islands must be large enough to hold pedestrians (especially children) and provide refuge.
- Lighting and signage are critical for child pedestrians.
- The high pedestrian volume may cause vehicle-pedestrian conflicts at the crosswalk, especially if drivers do not yield.
Key point: the single-lane roundabout is capacity-inadequate for 1,800 veh/h, and the pedestrian demand requires a carefully designed crossing with refuge islands and setback crosswalks.
2. Recommend a design or operational solution, and justify your choice.
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Recommendation: do not build a single-lane roundabout. Options:
- Two-lane roundabout with a signalized pedestrian crossing on the major approach (or a grade-separated pedestrian crossing) to handle 250 ped/h safely. A two-lane roundabout increases capacity but also increases pedestrian risk because of multiple lanes; a signalized crossing or a pedestrian hybrid beacon (HAWK) at the crosswalk can provide safe gaps.
- Retain the signalized intersection and improve pedestrian safety with leading pedestrian intervals (LPI), countdown signals, and refuge islands. This is often the safer choice for high pedestrian volumes, especially with school children.
- Grade-separated pedestrian crossing (overpass/underpass) if volumes justify it.
Justification: the single-lane roundabout cannot handle 1,800 veh/h, and the high pedestrian volume (250 ped/h) with children increases the risk of vehicle-pedestrian conflicts. A signalized intersection with LPI and refuge islands provides controlled, predictable crossings and adequate vehicle capacity. If a roundabout is desired, a two-lane design with a signalized or grade-separated pedestrian crossing is required.
Verification: conduct a capacity analysis (e.g., HCM roundabout methodology) and a pedestrian conflict analysis before finalizing the design.