New Study: Economic and Crash Impacts of Traffic Roundabouts: A Real-World Analysis

Friday, October 9, 2020

Modern roundabout intersections have received a lot of attention in recent years. While traffic professionals like roundabouts, the general public is more skeptical. Initial reactions to proposed roundabout intersections have been met with a variety of public comment ranging from unbridled enthusiasm to total disdain. Following construction, discussions typically evolve into identifying difficulties in traversing modern roundabout intersections and the resulting traffic crash experience once the new intersection is opened for public travel.

Supporting the popular opinion of traffic professionals, there are many pre-existing reports that say modern roundabouts provide several benefits when compared to other types of intersections, including:
1. Reducing injury severities
2. Reducing the cost of all crashes overall
3. Reducing peak-hour delay and congestion
4. Increases in Property Damage Only (PDO) crashes that gradually reduce in quantity

New MS2 Report

Until now, very little information has been available relating to the actual crash experiences at modern roundabout intersections, or the economic cost of these crashes. When severe traffic collisions do occur at intersections, the results have a lasting impact not only to those persons directly involved in the crash, but also to society which must bear the consequences and costs of responding to the incident, securing the location for the safety of others, and addressing legal, insurance and medical issues into the future.

MS2 has authored a new report that evaluates the “before and after” crash experiences and the social cost effects realized at twelve intersections in Washtenaw County where modern roundabouts replaced existing intersections. Most of the modern roundabout intersections constructed in Washtenaw County, Michigan were built in response to traffic crash mitigation needs at existing intersections. In some instances, however, they have been employed proactively where area growth predictions indicated future unsatisfactory intersection service level performances.

Our Methodology

In this study, we use the Traffic Crash Location System (TCLS) module of our Transportation Data Management System to identify traffic crash characteristics at each intersection prior to roundabout construction, then we compare those characteristics with post construction statistics. By comparing the crash characteristics with the economic crash cost before and after roundabout construction, we identify the net benefit or detriment achieved through implementing roundabouts. Each of the twelve roundabout intersections we studied were evaluated individually with respect to a before-and-after crash comparison, with specific focus on crash frequency, injuries, and costs. In addition, all the roundabouts were grouped together to establish an overall County-wide perspective showing how roundabouts are performing.

Our Findings

Overall, the implementation of the twelve roundabouts we studied has provided numerous benefits regarding post-construction economic and physical impacts. Not every roundabout, however, provided the expected benefits when analyzed individually, and one roundabout proved to be (how can we say this nicely?) a significant outlier.

The Best and Worst Performing Roundabouts

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When all 12 roundabouts are analyzed together, the four benefits described above are apparent. Furthermore, when the “outlier” roundabout was removed from the group analysis, the benefits provided by these roundabouts were even more apparent.


There may be other reasons for the installation of a roundabout that can’t be captured in the statistics of any report. For example, a roundabout may be installed at an interchange to reduce congestion and crashes on a nearby freeway. Roundabouts also provide traffic calming benefits, and other location-specific benefits that may justify their implementation. MS2 executed this study ourselves, with no sponsorship from any highway agency or special interest group. The data relied upon is primarily available to the public. Traffic crash data was gleaned from our TCLS software module with crash records provided by the Transportation Improvement Association of Michigan. The methods and procedures used, and the statistics documented in our report are the opinions of MS2 and do not represent the opinion of any highway or governmental agency within whose jurisdiction the roundabout intersection exists.

Learn More

Learn more and get your copy of this new MS2 report here.