Sight distance is the length of roadway ahead that is visible to the driver. The available sight distance on a roadway should be sufficiently long enough to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path.

Stopping sight distance is the sum of two distances:

Brake reaction distance –

the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied.

Braking distance –

the distance needed to stop the vehicle from the instant brake application begins on level terrain.

Approximately 90 percent of all drivers decelerate at rates greater than 11.2 ft/s

2

. Such decelerations allow the driver to maintain steering control during the braking maneuver on wet surfaces. Therefore,

11.2 ft/s

2

is recommended as the deceleration threshold for determining stopping sight distance.

In computing and measuring stopping sight distance, the height of the driver’s eye is estimated to be 3.5-ft and the height of the object to be seen by the driver is 2.0-ft, equivalent to the taillight height of a passenger car.

The calculated and design stopping sight distance are shown in .

The values given in represent stopping sight distance on level terrain. As a general rule, the sight distance available on downgrades is larger than on upgrades, therefore, corrections for grade are usually unnecessary.

The calculated corrections for these types of factors are given in . The computed distances for various speeds at the assumed conditions on level roadways are developed from the equations below.

$\text{SSD = 1.47Vt + 1.075}\frac{V^2}{a}$

Where:

SSD =

stopping sight distance, ft

V =

design speed, mph

T =

brake reaction time, 2.5 s

A =

deceleration rate, ft/s

2

When a highway is on a grade the equation for braking distance is modified to include the grade as shown below:

$d_B=\frac{V^2}{30\left[\left({\displaystyle \frac{a}{32.2}}\right)\pm G\right]}$

Where:

d

B

=

braking distance on grade, ft

V =

design speed, mph

A =

deceleration, ft/s

2

G =

grade, rise/run, ft/ft

The stopping sight distances for various grades are shown in these values are determined using the equation above.