PRSR 10-12-1994 - Laserfiche WebLink

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Feb. 8, 1983 BIKEWAYS -17- III E. SIGHT DISTANCE The equations are based on a height of eye of 3.75 feet and a height of object of zero inches. A mini- To insure safe and efficient operation, bicycle mum height of object is used here because such things facilities should be designed so that adequate sight distance as gravel on a surface can be dangerous to a cyclist. is provided for safe stopping, on crest vertical curves and horizontal curves. Figure B gives sight distance values for various vertical curve lengths, algebraic differences in grade and 1. Safe Stopping Sight Distance speed. Design values for stopping sight distance may be 3. Sight Distance at Horizontal Curves computed in the same manner as for a highway. Generally, there is no problem in attaining adequate stopping sight Where a high embankment or wall is on the in- distances on bicycle lanes and shared roadways because the side of a curve the sight distance for a bicyclist may not be roadway alignment usually has been designed to accom- sufficient. To check the horizontal sight distance for modate motor vehicle speed that are equal to or greater bicyclist, the bicycle has to be than bicycle � positioned 6 feet further cY speeds.There are exceptions, however,and the to the right than a motor vehicle. Figure C provides values stopping sight distance factor should be checked in lo- and formulas for sight distance on horizontal curves. cating such bicycle facilities. STOPPING SIGHT DISTANCE F. WIDTHS AND CLEARANCES FOR DOWNHILL GRADES Design Grade(%) The surfaced or operating width required for a bike- way is one of the primary considerations of design. Speed 0% 5% 10% 15% mph feet feet feet feet The minimum dimensions should include space re- 10 50 50 60 70 quired for the cyclist, allowance for lateral clearance to 15 85 90 100 130 obstructions, and allowance for clearance to other hazards. 20 130 140 160 200 The typical bicycle and rider dimensions are given in 5 175 200 230 300 Figure D. In addition to the physical space taken up by the 30 230 260 310 400 bicycle and rider the following maneuvering allowances and clearances are generally accepted: These values are based on a coefficient of skid resistance of 0.25, perception-reaction time of 2.5 seconds, height of eye of 3.75 feet and an object height of 6 inches. The Desirable Distance height of an adult bicyclist's eye will normally be greater T of Clearance feet than 3.75 feet but a lower object may be pertinent. Maneuvering Allowance — handlebars to edge (for wobble) 1.0 2. Sight Distance at Crest Vertical Curves — between bicycles(regardless of direction) 2.5 In order to provide a better sense of bikeway con- — between bicycles and pedestrians 2.5 tinuity and riding quality, the use of vertical curves is — between bicycles and motor recommended where changes in grade are encountered. vehicles 4.0(minimum) Sight distances at grade crests can be checked using the following equations: Lateral Clearance(static obstructions) As 2 — parked cars 2.0 L = 57 when S<L — curb drop-off 2.0 — utility poles, trees, hydrants, fences, railings, etc. 2.0 — soft shoulders 1.5 L= 2S — 750 when S > L — sloped drop-off 1.0 A — raised curb 1.0 Where S = sight distance in feet (based on design Vertical Clearances speed) 8.5 (10'desirable) L = length of vertical curve (in feet) illA = algebraic difference in grades in percent