SIGHT DISTANCE Spring 2020 Sight Distance Stopping Sight

SIGHT DISTANCE Spring 2020

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

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 (d 1) Braking Distance (d 2)

Stopping Sight Distance Braking Reaction Time Distance: Where, d 1 = 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, d 2 = Braking distance (ft) V = Design Speed (mph) a = deceleration rate (ft/s 2) Note: Deceleration rate for design: 11. 2 ft/s 2 Deceleration rate in emergency: 14. 8 ft/s 2

Stopping Sight Distance Braking Distance (with Grade): Where, d 2 = Braking distance (ft) V = Design Speed (mph) a = deceleration rate (ft/s 2) 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 (d 1) Braking Distance (d 2) 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=2. 5 s T=10. 2 s

Passing Sight Distance Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway Initial Maneuver Distance Clearance Distance Occupying Left-Lane Distance Opposing Vehicle Distance Old Method (no longer used)

Passing Sight Distance: Design Values

PASSING SIGHT DISTANCE • Important design considerations – – 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 considerations (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 PSD Object 2. 0 ft 3. 5 ft Object

Measuring Sight Distance