HomeMy WebLinkAboutPEDERSEN TOYOTA EXPANSION - PDP - PDP140007 - SUBMITTAL DOCUMENTS - ROUND 2 - VARIANCE REQUESTAugust 5, 2014
Marc Virata
Fort Collins Engineering
281 North College Avenue
P.O. Box 580
Fort Collins, CO 80522
Dear Marc:
This letter pertains to the intersection corner sight distance at the Mason/Site
Access Driveway intersection for the proposed Pedersen Toyota Expansion development
in Fort Collins. Mason Street is classified as a two-lane arterial street on the Fort Collins
Master Street Plan. The design speed on a two-lane arterial street is 50 mph. The existing
speed limit on Mason Street is 30 mph. According to Section 7.4.1.C.4 Corner Sight
Distance and Figure 7-16 in the Larimer County Urban Area Street Standards
(LCUASS), the intersection corner sight distance for this access is 1030 feet to the north
and south along Mason Street for a passenger car vehicle. The Mason/Site Access
intersection is within a compound curve on Mason Street. Therefore, the sight distance to
the north cannot be achieved.
Figure 7-16, LCUASS, was developed using A Policy on Geometric Design of
Highways and Streets, 1990 (AASHTO). The basis of this distance (1030 feet) is: if the
driver of a vehicle entering the major street sees an approaching vehicle at this specific
location (1030 feet away); and the approaching vehicle is at the design speed (50 mph);
and the entering vehicle accelerates at a normal rate to the design speed; then the
approaching vehicle will not be required to slow down at all before overtaking the entering
vehicle on the major street. According to one of the engineers on the committee that
prepared the LCUASS document, this basis was selected to keep traffic moving on the
major street at a constant rate with no slow down due to side street traffic. It assumes that
the driver entering the major street will only enter the major street when the approaching
vehicle is at or beyond 1030 feet from the access location. This is not practical, since the
driver entering the major street cannot possibly determine a distance of 1030 feet. Also,
based upon experience, the driver of the vehicle entering the major street will likely select a
gap at less than 1030 feet. This was recognized by AASHTO, and in the current (2011)
edition of A Policy on Geometric Design of Highways and Streets, AASHTO has eliminated
the basis of intersection sight distance described above. The time gap for 1030 feet is 14.0
seconds at the 50 mph design speed. This gap is more than 7 seconds longer than that
used in determining the operational level of service in the Highway Capacity Manual. The
gap used for sight distance determination should be similar to that used in the operations
analyses. It is recommended that Figure 7-16, LCUASS be updated to reflect the most
recent AASHTO design standards.
Since an update to Figure 7-16, LCUASS is not likely to occur prior to the
consideration/approval of the Pedersen Toyota Expansion development proposal, this
variance to the intersection sight distance standard is submitted. As mentioned earlier, the
Site Access Driveway is within a compound curve on Mason Street. As a two-lane arterial
street, the compound curves on Mason Street should have a centerline radius of 1075 feet.
The compound curves on Mason Street currently have a radius of 370 feet. Since Mason
Street is not designed as a standard two-lane arterial street, it is reasonable that the criteria
on Figure 7-16, LCUASS should not be used. Therefore, Exhibit 9-70, Stopping Sight
Distance for Turning Roadways from the Policy on Geometric Design of Highways and
Streets, AASHTO was used to determine the intersection sight distance for the Site Access
Driveway. Exhibit 9-70 from the cited AASHTO document is provided in Appendix A.
Since the compound curves in Mason Street are significantly substandard, a design
speed of 50 mph should not be used. Based upon the radius of 370 feet for the compound
curves in Mason Street, a "comfortable" speed for a vehicle to traverse the compound
curves would be approximately 33.5 mph (Exhibit 3-40, AASHTO/horizontal curve with no
super elevation). Therefore, a design speed of 40 mph was deemed appropriate to use
with Exhibit 9-70 from the cited AASHTO document. Using a design speed of 40 mph,
results in an intersection sight distance of 305 feet. A schematic of the intersection sight
distance at 305 feet is provided in Appendix B.
In light of the above information and analyses, it is respectfully requested that the
intersection sight distance variance from the sight distance reflected in Figure 7-16,
LCUASS be granted for the proposed Site Access Driveway for the Pedersen Toyota
Expansion development. This variance will not be detrimental to the public health, welfare,
and safety; will not reduce the design life of improvements; and will not cause the City of
Fort Collins additional maintenance costs. Thank you.
Sincerely,
Joseph M. Delich, P.E., PTOE
File: 1404 VAR LT01
APPENDIX A
AASHTO-Geometric Design of Highways and Streets
STOPPING SIGHT DISTANCE AT INTERSECTIONS FOR
TURNING ROADWAYS
General Considerations
The values for stopping sight distance as computed in Chapter 3 for open highway
conditions are applicable to turning roadway intersections of the same design speed. The values
from Chapter 3, together with the value for a design speed of 15 kmlh [10 mph], are shown in
Exhibit 9-70. These distances have been rounded upward to provide an increased factor of safety.
Metric US Customary
Design speed 15 20 30 40 50 60 70 Design speed 10 15 20 25 30 35 40 45
(km/h) (mph)
Stopping sight 15 20 35 50 65 85 105 Stopping sight 50 80 115 155 200 250 305 360
distance (rn) distance (ft)
Exhibit 9-70. Stopping Sight Distance for Turning Roadways
These sight distances should be available at all points along a turning roadway; wherever
practical, longer sight distances should be provided. They apply as controls in design of both
vertical and horizontal alignment.
Vertical Control
The length of vertical curve is predicated, as it is for open highway conditions, on sight
distance measured from the height of eye of 1,080 mm [3.5 ft] to the height of object of 600 mm
[2 ft]. Formulas shown in the section on "Crest Vertical Curves" in Chapter 3 apply directly.
Exhibits 3-75 and 3-76 show the relation between design speed, algebraic difference in gradient,
and length of crest vertical curve to provide stopping distance. The factor K is constant for each
design speed and the length of vertical curve is found by multiplying A, the algebraic differences
in percent of grades, by K.
For design speeds of less than 60 kmIh [40 mph], sag vertical curves, as governed by
headlight sight distances, theoretically should be longer than crest vertical curves. Lengths of sag
vertical curves are found by substituting the stopping sight distances from Exhibit 9-70 in the
formulas in the section on "Sag Vertical Curves" in Chapter 3. Because the design speed of most
turning roadways is governed by the horizontal curvature and the curvature is relatively sharp, a
headlight beam parallel to the longitudinal axis of the vehicle ceases to be a control. Where
practical, longer lengths for both crest and sag vertical curves should be used.
682
Mason is posted 30 mph and used a
design speed of 40 mph for sight distance.
APPENDIX B
SCALE: 1"=60'
SIGHT DISTANCE