Development Type: N/A
Services Provided: Roundabout Feasibility Study
Client: City of Mankato
The City of Mankato, and the whole MnDOT District 7 for that matter, has a long list of roundabouts installed over the past five years or so. The City, County, and State have a good track record of identifying candidates, analyzing their potential, and moving forward with design and construction when warranted and justified. Having been fortunate enough to consult on some of those projects, the City turned to us again to investigate the roundabout feasibility at a new intersection.
A feasibility study has similar elements to a traffic study, but the goal is to determine if a new traffic control will work as opposed to a new development. Our typical intersection review contains the following analyses:
–Traffic Volumes – examination of the traffic signal warrants and the balance between the legs of the intersection. Currently, no roundabout warrants exist as the Manual on Uniform Traffic Control Devices (MUTCD) details for stop signs and traffic signals. Many agencies therefore fall back on traffic signal warrants as a guide for when a roundabout may be appropriate. Roundabouts work best with equal flow between approaches. An intersection with a clear-cut majority of traffic on the mainline compared to the side-street may not be appropriate for roundabout control.
–Capacity – a simple analysis based on the Highway Capacity Manual to determine the necessary roundabout approach lanes to provide acceptable operations.
–Safety – a review of the crash history. Besides looking at the crash and severity rates, we examine the types of crashes to see if they would be correctable by a roundabout. For instance, roundabouts can eliminate left turn or right angle crash types due to their physical design.
–Concept Layout – a 2-D review of the possible roundabout footprint. We use general guidelines to size the roundabout appropriately for the approach lanes, traffic, design vehicle, and rough connections back to the public roads. The concept layout should identify right-of-way needs and readily apparent obstacles (nearby pond, intersection skew, adjacent access adjustments, etc.).
–Concept Cost – not a detailed estimate, but a rough magnitude of scale based on the concept layout.
Beyond these basics, we also included a review of existing operations, impacts to travel times, impacts to adjacent intersection operations (and spacing guidelines), and impacts to pedestrians and bicyclists.
Here’s what we found:
–The existing traffic volumes satisfy the Traffic Signal Warrant 2, Four-Hour Vehicular Volume, thresholds.
–The split between mainline and side-street ranges from approximately 80%/20% during the a.m. peak hour to 60%/40% during the p.m. peak hour and over the course of a typical day.
–Full access does not meet the spacing guidelines for Madison Avenue, which suggests the intersection should be a limited or partial access intersection only.
–The existing intersection has poor operations today. In particular, the p.m. peak hour has failing operations for the southbound left turn movement.
–A roundabout would provide acceptable operations today and into the short-term (year 2020) and long-term (year 2045) future.
–Potential issues at an adjacent major intersection in forecast year 2045 could stretch back to this proposed roundabout, impacting traffic operations more compared to the current side-street stop control or partial access as recommended by the spacing guidelines.
–The existing crash rate is above the average and critical crash rate thresholds, suggesting a safety issues at the intersection.
–Under roundabout control, the total number of crashes may increase, although with decreased severity due to the roundabout design and lower vehicle speeds.
–Existing mainline crossings, across a high-speed road, are un-striped and present a long exposure time due to the left and right turn lanes.
–The proposed roundabout would provide shorter pedestrian crossing distances and crossings of one direction of traffic at a time. However, the total pedestrian travel time may increase due to indirect paths to the crossings and having the crossings set back from the intersection.
–Roundabouts are more difficult for visually impaired pedestrians to cross.
–Option for bicyclists to travel through the intersection as a vehicle or as a pedestrian.
–The concept roundabout footprint has right-of-way and temporary easement needs, but major developments are avoided.
–The concept cost estimate for a proposed roundabout is $678,000. This estimate includes construction costs and other costs such as landscaping, utility adjustments, and lighting.
Based on the benefits and drawbacks, we concluded that a roundabout is indeed a feasible option for the Madison Avenue/Haefner Drive intersection with trade-offs in operations for vehicles and pedestrians that should be weighed in a final decision.