When planning a new roundabout, a thorough traffic study is the foundation of a successful design. Unlike traditional signalized intersections, roundabouts rely on yield control and geometric features to manage traffic flow. Without a robust analysis, engineers risk creating a facility that fails to deliver expected safety and capacity improvements. A comprehensive traffic study provides the data and insights needed to determine whether a roundabout is appropriate, what geometry and lane configuration to use, and how it will perform over its design life. This article expands on the essential steps and considerations for conducting a traffic study at a proposed roundabout location, drawing on best practices from the Federal Highway Administration (FHWA) and the National Cooperative Highway Research Program (NCHRP).

Defining the Study Area

The first and often underestimated step is to clearly define the study area boundaries. The study area must extend beyond the immediate intersection to capture traffic patterns on adjacent roads and alternative routes that may be affected. A poorly defined boundary can lead to missed interactions such as corridor spillback or rerouting due to driver behavior changes. Typically, the study area includes the primary intersection and all approaches within 0.5 to 1 mile, as well as nearby intersections that may experience diversion or congestion.

Engineers should consider the network connectivity and potential for traffic to shift from parallel routes. For example, if a two-lane roundabout replaces a signalized intersection on a suburban arterial, the analysis must include upstream driveways and side streets that could feed or absorb traffic. Geographic information systems (GIS) and aerial imagery are useful for delineating the area. The study boundaries should be documented in the scope of work and approved by the overseeing agency.

Identifying Influence Zones

Within the study area, identify the influence zones: the region where traffic operations are directly altered by the roundabout. This includes queue lengths on each approach, the weaving areas between closely spaced roundabouts, and pedestrian crossing points. For multilane roundabouts, the influence zone may extend further due to higher speeds and longer deceleration distances. Use default values from the FHWA Roundabout Guide (500–800 feet on approaches) as a starting point, then adjust based on posted speed and grade.

Collecting Traffic Data

Accurate, current traffic data is the lifeblood of any traffic study. The data collection effort must capture vehicle volumes, turning movements, vehicle classification, speeds, and non-motorized user counts. The specific methods and duration depend on the project scope, budget, and seasonal variations. Automatic traffic recorders (ATRs) provide continuous counts for seven to fourteen days to capture day‐to‐day variability, while manual turning movement counts (TMCs) are typically recorded during peak periods (morning, midday, evening) on a typical weekday and, if relevant, a weekend day.

Turning Movement Counts

TMCs are critical for roundabout design because they determine the critical lane volumes and required number of entry lanes. Counts should be collected in 15‑minute intervals to allow analysis of peak hour factor (PHF). For a roundabout, the design hour volume (DHV) is usually the 30th highest hour of the year, often approximated as the 85th to 95th percentile peak hour during typical conditions. Use video cameras or radar‐based sensors to collect turning movements; manual methods are reliable for low‐volume sites but prone to error at busy multilane approaches.

If the proposed roundabout is part of a larger development, conduct counts during the same season and avoid holidays, school breaks, or construction periods that skew data. The Institute of Transportation Engineers (ITE) publishes default traffic data collection guidelines in its Traffic Engineering Handbook.

Vehicle Classification and Speed Data

Roundabout design is heavily influenced by the design vehicle (e.g., single‐unit truck, bus, WB‑67 semi‑trailer). Collect vehicle classification data to determine the percentage of heavy vehicles. Heavy vehicles require larger turning radii, wider circulatory roadways, and truck aprons. Speed data on approaches informs the required stopping sight distance and splitter island placement. Radar guns or portable traffic classifiers can provide spot speeds; collect at least 100 vehicles per approach during free‐flow conditions.

Pedestrian and Bicycle Counts

Non‑motorized users must be counted separately, especially at locations near schools, parks, or transit stops. Pedestrian volumes are used to design crosswalk treatments, refuge islands, and signalization if warranted. Bicycle volumes help determine whether a shared path or dedicated cycle track is needed. Counts should be done during the same peak periods as vehicular counts and during the same time of year to capture seasonal variations.

Analyzing Current Traffic Patterns

Once raw data is collected, the analysis phase begins. This step identifies existing congestion points, peak periods, and operational deficiencies. The key outputs are the peak hour factor (PHF), directional distribution (D), and the level of service (LOS) for each approach.

Peak Hour Factor and Directional Distribution

The PHF is the ratio of the peak 15‑minute volume to the total hourly volume divided by 4. A low PHF (e.g., 0.75) indicates a very peaked flow, which can cause short‐term queues that a roundabout must accommodate. Directional distribution (D) represents the proportion of total traffic entering from the major approach. For roundabouts, a high D (e.g., 70/30 split) can lead to unbalanced flows that require an additional entry lane on the heavy leg. Document these calculations for each approach for both AM and PM peaks.

Existing Level of Service and Queue Lengths

Evaluate the existing intersection (whether signalized or stop‑controlled) using Highway Capacity Manual (HCM) methodologies. Calculate control delay and 95th percentile queue lengths. This baseline is essential for comparing the proposed roundabout’s performance and for justifying the project. At congested signals, roundabouts often reduce delays by 30‑50% if designed correctly. However, if the existing intersection already operates near capacity with high volumes, a roundabout may not be the optimal solution. Document findings clearly in the study report.

Estimating Future Traffic Volumes

A roundabout is intended to operate effectively for 20+ years, so future traffic projections are mandatory. Growth rates should be based on historical trends, regional travel demand models, and planned land use developments. For greenfield sites or major new subdivisions, use trip generation rates from the ITE Trip Generation Manual or local traffic impact study guidelines.

Applying Growth and Land Use Factors

Apply a compound annual growth rate to base volumes. Typical rates for suburban areas range from 1% to 3% per year, while urban infill can see 0‑1%. Adjust for planned developments adjacent to the study area by adding the projected trips from each development, allocated to the roundabout approaches via a traffic assignment model. For large developments, a microsimulation model may be needed to capture peak spreading effects.

It is common to evaluate opening year (Year 1), design year (Year 20 or 25), and an interim year. For each scenario, calculate the design hour volume, PHF, and directional splits. Use the NACTO Urban Street Design Guide as a resource for context‑sensitive design assumptions.

Assessing Safety and Capacity

Safety evaluation is a dual process: first, analyze existing crash data to identify patterns and severity; second, predict the roundabout’s safety performance. The FHWA’s Interactive Highway Safety Design Model (IHSDM) or the Highway Safety Manual (HSM) provide crash modification factors (CMFs) for converting a signalized or stop‑controlled intersection to a roundabout. Typical CMFs show a 30‑50% reduction in total crashes and a 70‑90% reduction in fatal and injury crashes.

Crash Data Analysis

Obtain at least three years of crash records from the state or local agency. Categorize crashes by type (rear‑end, angle, sideswipe, pedestrian), severity (fatal, injury, property damage only), and time of day. A high proportion of angle and left‑turn crashes indicates that a roundabout would be particularly beneficial, as these conflict types are eliminated. Also note any geometric or sight distance deficiencies in the existing layout.

Capacity Analysis Using HCM Methods

Roundabout capacity is governed by gap acceptance theory. The HCM 6th edition provides capacity models for single‑lane and multilane roundabouts based on entry flow, circulating flow, and critical gap values. Use dedicated software such as SIDRA Intersection or the FHWA’s Roundabout Calculator to perform the capacity analysis. Key outputs include degree of saturation (v/c ratio), control delay, queue lengths, and level of service for each approach and the overall intersection.

For multilane roundabouts, check lane utilization and imbalance. If one entry lane carries significantly more traffic than the other, striped lane assignments or one‑lane entries may be needed. Capacity analysis should be performed for each design scenario (opening, design year, and future plus background traffic).

Modeling the Proposed Roundabout

Traffic simulation software provides a dynamic representation of how the roundabout will operate under varying conditions. Unlike analytical models, simulations can capture stochastic behavior, vehicle interactions, and queue propagation. The most common tools are VISSIM (PTV Group), Aimsun, and SIDRA (which also includes simulation). For simple single‑lane roundabouts, analytical methods may suffice, but for complex multilane or closely spaced roundabouts, microsimulation is strongly recommended.

Model Calibration and Validation

Calibrate the simulation model using field data: travel times, queue lengths, and approach speeds. Adjust driver behavior parameters such as desired speed distribution, gap acceptance, and car‑following sensitivity. The model should replicate observed conditions within a defined tolerance (e.g., 85th percentile speeds within 3 mph). Document the calibration process and results.

Performance Measures

Run simulations for at least 10 replications per scenario to account for randomness. Collect performance measures: average delay, maximum queue length, number of stops, and throughput. Compare these to the existing intersection and to the no‑build alternative. Also, evaluate the roundabout’s performance under incident conditions (e.g., lane blockage, heavy pedestrian crossing) to verify robustness.

Key Considerations Beyond Vehicular Traffic

A successful roundabout design must accommodate all users. The traffic study must address pedestrian, bicycle, emergency vehicle, and environmental concerns early in the process.

Pedestrian and Bicycle Accommodation

At roundabouts, pedestrians cross at splitter islands and often need refuge. The study should determine pedestrian crossing volumes and desired walking paths. For high‑volume crossings (more than 100 pedestrians per peak hour), consider a pedestrian hybrid beacon or signalized crossing on one approach. For bicycle accommodation, decide between riding through the roundabout as a vehicle (with appropriate lane positioning) or using a separate shared‑use path. The study must include bicycle speed and volume data to select the appropriate treatment. See the FHWA’s Bicycle and Pedestrian Guidance for Roundabouts.

Emergency Vehicle Access

Fire trucks and ambulances require mountable curb areas or truck aprons to navigate the roundabout. The traffic study should document the design vehicle for emergency responders (e.g., fire apparatus with 50‑ft turning radius) and verify that the swept path fits within the roundabout. For critical facilities like hospitals or fire stations, consider adding emergency pre‑emption or larger circulatory widths. Queue analysis should include the impact of an emergency vehicle passage on normal traffic.

Environmental and Community Impact

Evaluate noise levels, air quality, and stormwater runoff. Roundabouts often reduce idling and stop‑and‑go traffic, leading to lower emissions. Estimate the reduction in vehicle delay to calculate emission benefits using tools like MOVES or CALINE4. Stormwater management may require permeable pavement or bioretention within the splitter islands. Gather community feedback through public meetings and surveys; address concerns about aesthetics, safety, and construction disruptions. Incorporate input into the final design.

Conclusion

Conducting a traffic study for a proposed roundabout is a multi‑faceted process that demands careful data collection, rigorous analysis, and thoughtful consideration of all users. By systematically defining the study area, collecting accurate turning movement and classification data, projecting future volumes, and modeling performance with recognized software, engineers can confidently determine whether a roundabout is the appropriate solution. The safety benefits, capacity gains, and operational improvements that roundabouts provide are well documented, but only when the design is grounded in sound traffic engineering. A comprehensive study not only justifies the project but also ensures the roundabout performs as intended over its lifespan. Following the guidelines from FHWA, HCM, and NCHRP best practices will lead to a robust, defendable study that supports smarter, safer infrastructure decisions.