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SIGHT DISTANCE. Spring 2014. Sight Distance. Stopping Sight Distance. Decision Sight Distance. Passing Sight Distance. Note: A driver’s ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway. Example: Sight Distance.
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SIGHT DISTANCE Spring 2014
Sight Distance Stopping Sight Distance Decision Sight Distance Passing Sight Distance Note: A driver’s ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway
Stopping Sight Distance Definition: The available sight distance on a highway that allows a vehicle traveling near the design speed to stop before reaching a stationary object in its path Brake Reaction Time (d1) Braking Distance (d2)
Stopping Sight Distance Braking Reaction Time Distance: Where, d1 = BRT distance (ft) V = Design Speed (mph) t = Braking reaction time (s) Note: Usually 2.5 seconds is used as the BRT. However, some drivers can take as long as 3.5 seconds. The absolute minimum should be 1.64 seconds.
Stopping Sight Distance Braking Distance: Where, d2 = Braking distance (ft) V = Design Speed (mph) a = deceleration rate (ft/s2) Note: Deceleration rate for design: 11.2 ft/s2 Deceleration rate in emergency: 14.8 ft/s2
Stopping Sight Distance Braking Distance (with Grade): Where, d2 = Braking distance (ft) V = Design Speed (mph) a = deceleration rate (ft/s2) G = Grade in percent (- for downgrade, + for upgrade)
Stopping Sight Distance Example: Calculate the braking distance for a highway section characterized with a design speed of 60 mph and a 8% downgrade
Decision Sight Distance Definition: The available sight distance on a highway needed to detect an unexpected or otherwise difficult-to-perceive information in a roadway environment Decision Time (d1) Braking Distance (d2) Same as SSD
Decision Sight Distance General Guidelines: Stop on Rural Highway: 3.0 s Stop on Urban Highway: 9.1 s Speed/Direction Change on Rural Highway: 10.2 - 11.2 s Speed/Direction Change on Urban Highway: 14.0 – 14.5 s Other Areas: Interchanges Toll Plaza Note: In some instances, DSD may not be economically feasible. In these cases, the use of suitable traffic control devices should be provided
T=10.2 s T=2.5 s
Passing Sight Distance Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway Sum of four distances: d1 – Distance traversed during the perception and reaction time + acceleration to the point of encroachment d2 – Distance traveled while the passing vehicle occupies the left lane d3 – Distance between the passing vehicle at the end of its maneuver and the opposite vehicle d4 – Distance traversed by an opposing vehicle for 2/3 of the time the passing vehicle occupies the left lane
Passing Sight Distance Initial Maneuver Distance Clearance Distance Opposing Vehicle Distance Occupying Left-Lane Distance
PASSING SIGHT DISTANCE • Important assumptions • The overtaken vehicle travels at uniform speed • The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section • The passing vehicle accelerates during the maneuver and its average speed is 10 mph higher than the overtaken vehicle • When the passing vehicle returns to its lane, there is suitable clearance length between it and the oncoming vehicle in the other lane
Passing Sight Distance Initial Maneuver Distance (d1): Where, ti = time of initial maneuver (s) v = average speed of passing vehicle (mph) a = average acceleration rate (ft/s2) m = difference in speed between passing and passed vehicles (mph) Note: Average time for ti is between 3.6 and 4.6 seconds Average acceleration a is between 1.4 and 1.5 ft/s2
Passing Sight Distance Distance for Occupying Left Lane (d2): Where, v = average speed of passing vehicle (mph) t2 = time passing vehicle occupies the left lane (s) Note: Average time for t2 is between 9.3 and 10.4 seconds
Passing Sight Distance Clearance Distance (d3): Usually varies between 100 to 300 ft Opposite Vehicle Distance (d4): Assume to be 2d2/3 Note: Opposite vehicle assumed to travel at the same speed as the passing vehicle
Passing Sight Distance: Design Values Exhibit 3-7
Passing Sight Distance ti=4.5; t2=10.5 ti=4.5; t2=9.3 ti=3.6; t2=9.3
PASSING SIGHT DISTANCE • Important design consideration • The range of speeds are affected by traffic volume • Low volume – few vehicles need to be passed • High volume – few opportunities to pass • Should provide as many as passing sections as possible • Should not be confused with other warrants used for designing no passing zones
PASSING SIGHT DISTANCE • Important design consideration (cont’d) • Overtaking a vehicle is easier for downgrade segments • Need to provide longer passing sight distance for upgrade • In mountainous terrain, it may be more economical to provide for a “climbing lane” • The number of passing opportunities greatly affects the capacity of your facility
Criteria for Measuring Sight Distance Driver Eye Height Passenger Car: 3.5 ft Large Trucks: 5.9 to 7.9 ft SSD Object 2.0 ft PSD Object 3.5 ft Object