Sight Distance is a length of road surface which a particular driver can see with an acceptable level of clarity. Sight distance plays an important role in geometric highway design because it establishes an acceptable design speed, based on a driver's ability to visually identify and stop for a particular, unforeseen roadway hazard or pass a slower vehicle without being in conflict with opposing traffic. As velocities on a roadway are increased, the design must be catered to allowing additional viewing distances to allow for adequate time to stop. The two types of sight distance are (1) stopping sight distance and (2) passing sight distance.
Stopping Sight Distance
Forces acting on a vehicle that is braking
Stopping Sight Distance (SSD) is the viewable distance required for a driver to see so that he or she can make a complete stop in the event of an unforeseen hazard. SSD is made up of two components: (1) Braking Distance and (2) Perception-Reaction Time.
For highway design, analysis of braking is simplified by assuming that deceleration is caused by the resisting force of friction against skidding tires. This is applicable to both an uphill or a downhill situation. A vehicle can be modeled as an object with mass sliding on a surface inclined at angle .
While the force of gravity pulls the vehicle down, the force of friction resists that movement. The forces acting this vehicle can be simplified to:
- = object’s weight,
- = coefficient of friction.
Using Newton’s second law we can conclude then that the acceleration () of the object is
Using our basic equations to solve for braking distance () in terms of initial speed () and ending speed () gives
and substituting for the acceleration yields
Ample Stopping Sight Distance
For angles commonly encountered on roads, and , where is called the road’s grade. This gives
Using simply the braking formula assumes that a driver reacts instantaneously to a hazard. However, there is an inherent delay between the time a driver identifies a hazard and when he or she mentally determines an appropriate reaction. This amount of time is called perception-reaction time. For a vehicle in motion, this inherent delay translates to a distance covered in the meanwhile. This extra distance must be accounted for.
For a vehicle traveling at a constant rate, distance covered by a specific velocity and a certain perception-reaction time can be computed using simple dynamics:
Finally, combining these two elements together and incorporating unit conversion, the AASHTO stopping sight distance formula is produced. The unit conversions convert the problem to metric, with in kilometers per hour and in meters.
A Note on Sign Conventions
if going downhill
if going uphill, where G is the absolute value of the grade
Passing Sight Distance
Passing Sight Distance (PSD) is the minimum sight distance that is required on a highway, generally a two-lane, two-directional one, that will allow a driver to pass another vehicle without colliding with a vehicle in the opposing lane. This distance also allows the driver to abort the passing maneuver if desired. AASHTO defines PSD as having three main distance components: (1) Distance traveled during perception-reaction time and accleration into the opposing lane, (2) Distance required to pass in the opposing lane, (3) Distance necessary to clear the slower vehicle.
The first distance component is defined as:
- = time for initial maneuver,
- = acceleration (km/h/sec),
- = average speed of passing vehicle (km/hr),
- = difference in speeds of passing and impeder vehicles (km/hr).
The second distance component is defined as:
- = time passing vehicle is traveling in opposing lane,
- = average speed of passing vehicle (km/hr).
The third distance component is more of a rule of thumb than a calculation. Lengths to complete this maneuver vary between 30 and 90 meters.
With these values, the total passing sight distance (PSD) can be calculated by simply taking the summation of all three distances.
Example 1: Stopping Distance
A vehicle initially traveling at 66 km/h skids to a stop on a 3% downgrade, where the pavement surface provides a coefficient of friction equal to 0.3. How far does the vehicle travel before coming to a stop?
Example 2: Coefficient of Friction
A vehicle initially traveling at 150 km/hr skids to a stop on a 3% downgrade, taking 200 m to do so. What is the coefficient of friction on this surface?
Example 3: Grade
What should the grade be for the previous example if the coefficient of friction is 0.40?
Thus the road needs to be a 4 percent uphill grade if the vehicles are going that speed on that surface and can stop that quickly.
Example 4: Crash Reconstruction
You are shown an crash scene with a vehicle and a light pole. The vehicle was estimated to hit the light pole at 50 km/hr.
The skid marks are measured to be 210, 205, 190, and 195 meters. A trial run that is conducted to help measure the coefficient of friction reveals that a car traveling at 60 km/hr can stop in 100 meters under conditions present at the time of the accident. How fast was the vehicle traveling to begin with?
First, Average the Skid Marks.
Estimate the coefficient of friction.
Third, estimate the unknown velocity
Example 5: Compute Stopping Sight Distance
Determine the Stopping Sight Distance from Example 4, assuming an AASHTO recommended perception-reaction time of 2.5 seconds.
If the coefficient of friction is 0 (zero) and the grade is 0, how long does it take a moving vehicle to stop?
Note, the design conditions for roads are wet, i.e. a lower coefficient of friction
- - stopping (sight) distance (m)
- - perception reaction distance (m)
- - braking distance (m)
- - passing distance (m)
- - initial speed (km/h)
- - perception/reaction time (seconds)
- - AASHTO stopping friction coefficient (dimensionless)
- - roadway grade (dimensionless)
- SSD : Stopping Sight Distance
- PSD : Passing Sight Distance
- PRT : Perception-Reaction Time
Standards and Practices
AASHTO Recommended Friction Coefficients
||Coefficient of Skidding Friction (f)