HomeMy WebLinkAboutWPO200500013 Study Road Plan and Comps. 2005-03-21 • 1740 Lambs Road
• Charlottesville,Virginia 22901
• Phone:(434)978-7879
• Fax:(434)973-0249
e-mail:Tmuncaster@earthlinknet
Muncaster Engineering
March 21, 2005
Mr. Mike Boggs,President
Haley Chisholm&Morris, Inc.
P.O. Box 550
Earlysville,VA 22936-0550
Re: Stopping Sight Distance at 15 mhp
Dear Mr.Boggs:
At your request I have evaluated the necessary stopping sight distance for 15 mph design speed.
Stopping sight distance is the sum of reaction distance,the distance traveled by a vehicle from the
time a driver sights an object to the time the brakes are applied,and braking distance.
2.5 seconds is the commonly accepted reaction time used to compute reaction distance. At 15 mph a
vehicle travels 2.5 sec X 15 mi/hr X 5280 ft/mi X 1 hr/3600 sec=55 feet in 2.5 seconds.
The braking distance is a function of the vehicle deceleration rate and road design speed:
d=1.075 V2/a
d=braking distance(ft)
V=design speed(mph)
a=deceleration rate 11.2 ft/sec2
At 15 mph: d=1.075 X 152/11.2=21.6 feet
The total stopping distance is 55+21.6=76.6 feet
Rounding for design, 80 feet is the required stopping distance for 15 mph design speed.
K-values for sag vertical curve design are developed using 3.5'height of eye and 2.0'height of
object with:
K= SSD2/(400+3.5 X SSD)
SSD= stopping sight distance
At 15 mph: 802/(400+3.5 X 80)= 9.4(rounded to 10 for design)
The ultimate reference for this information is AASHTO's A Policy on Geometric Design of
Highways and Streets. I hope this provides the information you required.
Sincerely,
af/ 7t,m u 7Xscneaatos,
W.Thomas Muncaster, Jr.,P.E.
Bongs 3-21-05