SIGHT DISTANCE Spring 2015

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

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

Stopping Sight Distance: Design Values

Stopping Sight Distance: Design Values

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

Decision Sight Distance: Design Values

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: Design Values

PASSING SIGHT DISTANCE 10 30 50 70 Mph

PASSING SIGHT DISTANCE Based on HSM – Multiple Passing Zones

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

Measuring Sight Distance

Passing Sight Distance Previous Detailed Calculations

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

Passing Sight Distance: Design Values