Roundabouts vs Traffic Lights: Which Is Better?
The UK has 25,000 roundabouts; the US barely has 10,000. France converts 500 intersections per year. The data is clear on safety and surprisingly nuanced on throughput. Here's what the research says.
1. How Each Design Works
Traffic signals rotate right-of-way through phases. A 4-arm signal has typically 2–4 phases, each giving green to a subset of movements. Vehicles stop during red phases. Cycle time: 60–120 s. Throughput is maximised when green splits match demand ratios.
Roundabouts give priority to traffic already circulating. Entering vehicles yield, finding natural gaps. There is no red phase — vehicles continuously flow through at low speed (15–25 mph). Conflict points are reduced because all traffic moves in one direction.
Key geometry: a single-lane roundabout with inscribed circle diameter of 30–40 m (modern) vs 60–80 m (older British roundabouts). Turbo roundabouts use raised lane dividers to prevent lane-changing within the circle.
2. Safety: The Numbers
(IIHS data: US conversions from signals/stop signs to modern roundabouts)
Why the dramatic safety improvement? Conflict points. A conventional 4-way intersection has 32 vehicle-to-vehicle conflict points, including 16 crossing (right-angle) conflicts. A single-lane roundabout has only 8 conflict points — all merging/diverging, none crossing. The geometry also forces low speeds: entry deflection requires deceleration to 15–25 mph. At 20 mph, pedestrian fatality risk is 5%; at 40 mph it is 45%.
3. Throughput & Capacity
| Design | Capacity (veh/h) | Average Delay | Best When |
|---|---|---|---|
| Single-lane roundabout | 20,000–26,000 (entry sum) | 5–15 s | Balanced flows, moderate volumes |
| 2-lane roundabout | 40,000–50,000 | 8–20 s | Higher volumes, up to 6 arms |
| 2-phase signal | 25,000–35,000 | 20–45 s | 2-way, dominant through movement |
| 4-phase signal | 20,000–30,000 | 35–60 s | Complex turning, unbalanced flows |
Key insight: roundabouts win at low-to-moderate volumes because there's no red time. But at very high volumes or heavily imbalanced flows (e.g. 3:1 ratio between arms), signalised intersections can be more efficient because they can allocate green time proportionally. Roundabouts give equal treatment to all arms — a heavily loaded arm can block the others.
4. Fuel & Emissions
At a traffic signal, vehicles decelerate, idle, then accelerate — the least fuel-efficient driving pattern. Studies show roundabout conversions reduce:
- CO₂: 15–45% reduction (Hallmark & Mueller, Kansas State, 2004–2015 meta-analysis)
- CO: 20–40% reduction
- NOₓ: 15–30% reduction
- Fuel consumption: 23–34% reduction at low-moderate volumes
The savings come from continuous flow: roundabout traffic maintains 15–25 mph through the intersection rather than stopping and restarting from 0 mph. Idling at a red light burns 0.5–1.0 L/hour of fuel for no useful movement.
5. Pedestrians & Cyclists
Roundabouts are mixed for pedestrians:
- Advantage: Crossing is split into stages (splitter islands). Each crossing is only one lane wide. Vehicles travel slowly. Crash severity is lower.
- Disadvantage: No pedestrian phase — must find a gap. Multi-lane roundabouts are problematic for visually impaired pedestrians who rely on audible cues from stopped traffic. In the US, the ADA considers multi-lane roundabouts a barrier.
Solutions: raised pedestrian crossings with tactile paving, Rectangular Rapid Flashing Beacons (RRFB) at crosswalks, or hybrid signal-controlled crossings on high-pedestrian approaches.
For cyclists: dedicated cycle paths with separate priority crossings (Dutch-style roundabouts) are the safest. Shared-lane roundabouts require cyclists to "take the lane" in the circulatory roadway — this works in single-lane roundabouts but is dangerous in multi-lane ones.
6. When Each Design Wins
| Scenario | Best Choice | Why |
|---|---|---|
| Suburban 4-way, 15k veh/day | Roundabout | Huge safety gains, lower delay, no signals to maintain |
| Urban arterial, 50k veh/day | Signal | High volume, coordinated green wave possible |
| Irregular geometry (5+ arms) | Roundabout | Signals get complex; roundabouts naturally handle any angles |
| Near school/hospital (heavy ped) | Signal | Protected pedestrian phase for vulnerable users |
| Highway off-ramp diamond | Roundabout | Continuous flow, eliminates left-turn crashes |
| Grid network with progression | Signal | Coordinated signals enable green wave at 35 mph |
7. Simulating the Difference
Our Traffic Simulation lets you compare both designs interactively. Parameters you can adjust:
- Volume per approach: 100–1,500 veh/h per arm
- Pedestrian rate: 0–200 peds/h per crossing
- Signal timing: Cycle length, green splits, offset
- Roundabout lanes: 1 or 2, with or without bypass slip lanes
The simulation uses a car-following model (Intelligent Driver Model) and gap-acceptance logic for roundabout entry. It tracks delay, throughput, queue length, and CO₂ in real time. Try setting balanced 800 veh/h per arm and compare: the roundabout typically shows 60% less average delay in this regime.