HomeMy WebLinkAbout2014-06-23 PACKET 09.2.Washington
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.- County
Public Works Department
Donald J. Theisen, P.E.
Director /County Engineer
Wayne H. Sandberg, P.E.
Deputy Director /Assistant County Engineer
MEMORANDUM
Date: June 16, 2014
To: John McCool, City of Cottage Grove
Jennifer Levitt, City of Cottage Grove
From: Joe Gustafson, Washington County Traffic Engineer
Re: Lane Configuration and Approach Slopes to Roundabouts
Washington County staff were notified that the City of Cottage Grove Planning Commission
requested additional information on how lane configurations are determined for roundabouts on
Washington County highways. As with other intersection types, the number of lanes
constructed on any given intersection approach depends on the current and anticipated traffic
volumes, which results in a variety of different lane configurations. The most recent roundabout
constructed in Cottage Grove, at the intersection of County Highway 22 (70 St) and County
Highway 19 (Keats Ave S) has only one approach lane from all directions, as this configuration
is expected to handle projected traffic demands. A similar single -lane configuration is proposed
for the intersection of County Highway 13 (Radio Drive) at County Highway 20 (Military Road),
with the ability to add lanes in the future as needed.
Multi -lane approaches are constructed where a single approach lane would not be expected to
adequately handle traffic demands. The two roundabouts constructed on County Highway 19 in
Woodbury in 2013 each include two approach lanes on the north and south approaches, but
only a single approach lane on the east and west approaches. The roundabout on County
Highway 13 (Radio Drive) in Woodbury was originally constructed with two lanes on all
approaches, but because traffic growth fell short of expectations, the lane configuration was
changed on the east and west approaches such that only one through lane was provided. A
similar design is envisioned for a proposed roundabout at County Highway 22 (70 St) and
Jamaica Ave. Providing more lanes than needed at roundabouts increases the potential for
driver mistakes but does little to improve operations or reduce delays.
Approach slopes to roundabouts are treated similarly to stop signs or traffic signals, as both are
potential stop conditions for drivers. Even when the approaching roadways are on grade, it is
often possible to reduce the grade at the roundabout itself. For example, the proposed
roundabout on County Road 20 (Military Road) will have approaches that flatten to less than
1.25% grade at the crosswalks and yield sign.
CC: Nik Costello, Wayne Sandberg
11660 Myeron Road North, Stillwater, Minnesota 55082 -9573
Phone: 651 - 430 -4300 • Fax: 651 - 430 -4350 • TTY: 651 - 430 -6246
www.co.washington.mn.us
Equal Employment Opportunity / Affirmative Action