Agenda - Parks and Recreation Commission - 01/12/1995

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Feb. 8, 1983 i I I I i I IE. SIGHT DISTANCE To insure safe and i efficient operation, bicycle I facilities should be designed [o that adequate sight distance ii provided for safe stop ~l~ng, on crest vertical curves and horizontal curves. 1. Safe Stopping Sight Distance Design values for stDpping sight distance may be computed in the same manner as for a highway. Generally, there is no problem in attaining adequate stopping sight distances on bicycle lanes aMI shared roadways because the roadway alignment usually ~ihas been designed to accom- modate motor vehicle speec[ that are equal to or greater than bicycle speeds. There an~ exceptions, however, and the stopping sight distance factor should be checked in lo- caring such bicycle facilities. sTOPPING SIOHT DISTANCE FOR DOWNI~ILL GRADES Designrr Grade (%) Speed mph feet f~tet feet feet 10 50 ~0 60 70 ~5 85 ~)0 100 20 ~50 ~0 ~60 20O 25 175 2~0 230 300 30 ~0 ~0 310 4~ Thes~ values are based on aicoefficient of skid resistance of 0.25, percepdon-reactionftirne of 2.5 seconds, height of eye of 3.75 feet and an ~bject height of 6 inches. The height of an adult bicyclist'si eye will normally be greater than 3.75 feet but a lower object may be pertinent. 2. Sight Distance at Cre~t Vertical Curves In order to provide a better sense of bikeway con- tinuity and r/ding quality, ~the use of vertical curves is recommended where change~ in grade are encountered. Sight distances at grade creW. ts can be checked using the following equations: L= AS2 whenS<L 750 ~ L = 2S - 750. when S > L A Where S = s~ght distanceiin feet (based on design speed) L = length of vertical curve (in feet) A = algebraic difference in grades in percent BIKEWAYS -17. The equations are based on a height of eye of 3.75 feet and a height of object of zero inches. A mini. mum height of object is used here because such things as gravel on a surface can be d~gerous tO a CyCkiSt. Figure B c~ves sight distance values for various vertical curve lengths, algebraic differences in grade and speed. 3. Sight Distance at Horizontal Curves Where a high embankment or wall is on the in- s/de of a curve the sight distance for a bicyclist may not be ~ufficient. To check the horizontal sight distance for bicyclist, the bicycle has to be positioned 6 feet further to the right than a motor vehicle. Figure C provides values and form-l~ for sight distance on horizontal curves. F. WIDTHS AND CLEARANCES The surfaced or operating width required for a bike- way is one of the primary considerations of design. The minimum dimensions should include space re- quired for the cychst, allowance for lateral clearance to obstructions, and allowance for clearance to other hazards. The typical bicycle and rider dimensions are given in Figure D. In addition to the physical space taken up by the bicycle and rider the following maneuvering allowances and clearances are generally accepted: Type of Clearance Maneuve~g Allowance - handlebars to edge (for wobble) - between bicycles (regardless of direction ) - between bicycles and pedestrians - between bicycles and motor vehicles Lateral Clearance (static obstructions) - parked cars - curb drop-off - utility poles, trees, hydrants, fences, railings, etc. - soft shoulders - sloped drop-off - raised curb Desirable Distance feet 1.0 2.5 2.5 4.0 (minimum) 2.0 2.0 2.0 1.5 1.0 1.0 Vertical Clearances 8.5 (10' desirable)