HomeMy WebLinkAbout19.0 108 West Babcock Vision Triangle Support and Analysis
619 Milestone Drive
Belgrade, MT 59714
E5EngineeringPLLC@gmail.com
(406) 209-3960
November 5, 2024
Melanie Dyba, PE
Bridger Engineers, Inc.
2150 Analysis Drive, Suite B
Bozeman, MT 59718
Re: 108 West Babcock Street – Bozeman, Montana | Vision Triangle Deviation Support & Analysis
E5 Engineering, PLLC Project No. 24-0107
Melanie:
In support of the deviation request from the City of Bozeman’s street vision triangle criteria as per Bozeman
Municipal Code (BMC) Section 38.400.100.E., the following information delineates the existing characteristics and
traffic control at the intersection of South Willson Avenue and West Babcock Street as well as guidance from the
American Association of State Highway Transportation Officials (AASHTO) publication A Policy on the Geometric
Design of Highways and Streets, 2018 7th Edition (2nd Printing) plus the Urban Street Design Guide published by
the National Association of City Transportation Officials (NACTO).
Existing Intersection Characteristics
The existing intersection of South Willson Avenue and West Babcock Street are each classified as minor arterial
roadways by both the Montana Department of Transportation as well as the Bozeman Transportation Plan dated
April 25, 2017. South Willson Avenue is a two-way street with single travel lanes in each direction with a dedicated
southbound left-turn lane at its intersection with West Babcock Street as well as a dedicated northbound right-turn
lane. West Babcock Street is an eastbound one-way street with two travel lanes. The intersection of the two streets
is controlled by a traffic signal. Each street has a posted speed limit of 25 miles per hour (mph).
Intersection
Traffic
Control Approach
Lane
Configuration
Bike
Lane
Pedestrian Accommodations
Crossing Marked Crosswalk
South Willson Avenue
&
West Babcock Street
EB
NB
SB
From observations and local experience, the existing traffic signal is not put into flashing operation during off-peak
periods. If there is an issue with the signal that trips the conflict monitor or a power outage, the signal should
operate as an all-way stop controlled intersection. Parking is prohibited along the west side of South Willson Avenue
from approximately 54-feet north of the centerline of West Babcock Street to roughly 50-feet south of the centerline
of West Olive Street. Parking is prohibited for approximately 70-feet along the south side of West Babcock Street as
measured from its intersecting curb line with the west side of South Willson Avenue.
108 West Babcock Street – Bozeman, Montana | Vision Triangle Deviation Support & Analysis November 5, 2024
108-W-Babcock_Vision-Triangles-Summary_E5_20241105 Page 2 of 2
AASHTO Guidance
A Policy on Geometric Design of Highways and Streets, 2018 7th Edition (2nd Printing) notes the following for
intersections with traffic signal control (Section 9.5.3.4):
“At signalized intersections, the first vehicle stopped on one approach should be visible to the driver
of the first vehicle stopped on each of the other approaches. Left-turning vehicles should have
sufficient sight distance to select gaps in oncoming traffic and complete left turns. Apart from these
sight conditions, there are generally no other approach or departure sight triangles needed for
signalized intersections. Signalization may be an appropriate crash countermeasure for higher
volume intersections with restricted sight distance that have experienced a pattern of sight-distance
related crashes.”
The first vehicle stopped on each approach at the intersection would be visible to a driver of the first vehicle stopped
on each of the other approaches, and there are not presently oncoming vehicle conflicts for eastbound vehicles on
West Babcock Street. No reported crashes were noted from a review of the crash history of the intersection of South
Willson Avenue and West Babcock Street using data provided from the MDT crash database. Public crash
information for the period 2019 through 2023, the latest five years available, was reviewed. Limited data is
available through the public crash information provided by MDT. Additional crash information such as type; weather,
roadway, and lighting conditions; number of vehicles, pedestrian, or bicyclist involvement; age and gender of drivers;
and other potentially contributing conditions can be obtained through a review of the specific crash reports.
NACTO Guidance
An additional reference, the Urban Street Design Guide from 2013, notes “Wide corners with large sight triangles
may create visibility, but in turn may cause cars to speed through the intersection, losing the peripheral vision they
might have retained at a slower and more cautious speed.” This publication also provides additional guidance on
enhancing visibility of non-motorized users and sight distance improvements through parking setbacks.
Summary
It is recommended to utilize the AASHTO criteria for signalized intersections (Section 9.5.3.4) that “there are
generally no other approach or departure sight triangles needed for signalized intersections,” considering existing
signal operations, lack of a documented crash experience history, and the one-way traffic flow on West Babcock
Street. Visibility of pedestrians on the intersecting sidewalks at the southwest corner of the intersection of South
Willson Avenue and West Babcock Street should be maintained through parking setbacks.
· Additional supporting guidance from NACTO provides reasoning for a balanced approach to sight
triangles to enhance safety for non-motorized users through retaining slower and more cautious
speeds for vehicles traveling through the intersection.
· Parking setbacks should match other similar, signalized intersections in the downtown area.
(One comparable intersection is North Willson Avenue and West Mendenhall Street.)
If you have any questions or concerns regarding the analyses, please do not hesitate to contact me.
Regards,
Tom Eastwood, PE, PTOE
Owner/President