HomeMy WebLinkAbout16055-CO-Renton-Road Evaluation-2018-01-15
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January 15, 2018
ELECTRONIC DELIVERY
Ann Fowler
Development Engineering
City of Renton
Community and Economic Development
1055 South Grady Way
Renton, WA 98057
RE: Concrete Retaining Wall
Plat of Allura at Tiffany Park
City of Renton File No. C17004253
BCE Job No. 16055
Dear Ann:
The City of Renton has requested Barghausen Consulting Engineers, Inc. evaluate the need for additional
roadside safety elements with respect to the proposed retaining wall and traffic barrier to be constructed
along that section of Road A within the Allura at Tiffany Park development adjacent to Tract K.
Proposed Road A is a Residential Access road as classified by the City of Renton. The road is located
within an internal loop for the subdivision and is not considered a "through" street that would connect to
other neighborhoods.
The speed limit for this local access residential street is 25 mph. The expected ADT is less than 750. Due
to the topography, adjacent critical areas, and design grades, a cast-in-place concrete wall has been
designed to mitigate the grade transition between the new residential road and the adjoining critical area in
Tract K.
Road A has been designed to provide adequate sight distance and is illuminated with Public Street Lights.
The retaining wall has been designed with a reinforced concrete "traffic barrier" on top of the retaining wall.
The traffic barrier will have a height of 2 feet 8 inches above the proposed sidewalk grade. Safety fencing
for pedestrians will be provided on top of the traffic barrier.
To evaluate the need for additional roadside safety elements we have consulted the AASHTO Roadside
Design Guide, 4th Edition 2011. Chapter 3, Table 3-1 provides suggested Clear-Zone Distances from
"Through Traveled Lanes". For design speeds less than 40 mph with ADT under 750 and level slopes, the
suggested Clear Zone is between 7 and 10 feet.
The half street road section adjacent to the entire length of the retaining wall is designed to have a 12 foot
wide asphalt "through traveled lane", 1.5 feet of vertical concrete curb and gutter, 2 feet of landscape
planter strip, and 5.5 feet of concrete sidewalk abutting the concrete traffic barrier on top of the retaining
wall.
The above provides a total of 8 feet of physical horizontal separation (Clear Zone) between the face of curb
and the face of the traffic barrier and a total of 9 feet from the edge of the "through travelled lane". This is
well within the guidelines provided in AASHTO Section 3.1.
Ann Fowler
Development Engineering
City of Renton -2- January 15, 2018
Given that Road A is within a local access loop in a residential neighborhood, with very low traffic volumes,
a speed limit of 25 MPH, adequate illumination and sight distance, and a Clear Zone of 8-9 feet between
the edge of the "through traveled lane" and the face of the traffic barrier which is well within the suggested
guidelines provided by AASHTO, it is our professional opinion that no additional roadside safety elements
are warranted.
Sincerely,
Barry J. Talkington, P.E.
Senior Project Engineer
BJT/ns
16055c.030
enc: As Noted
cc: Lisa Cavell, MainVue WA LLC
Stephannie Karlsson, MainVue WA LLC
Thomas A. Barghausen, Barghausen Consulting Engineers, Inc.
1/15/18