Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source, recognize the source, select an appropriate speed and path, and initiate and complete the required maneuver safely and efficiently.

Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than stopping sight distance. shows recommended decision sight distance values for various avoidance maneuvers

Examples of situations in which decision sight distance is preferred

include the following:

Locations along the roadway where a driver has stopping sight distance but not the extra response time provided by decision sight distance is identified as a

“reduced decision zone.”

Avoid placement of intersections within a reduced decision zone by relocating the intersection or by changing the grades to reduce the length of the reduced decision zone.

Passing sight distance is the distance required by a driver to make a passing maneuver without cutting off the passed vehicle and before meeting an opposing vehicle. Therefore, passing sight distance is applicable to two-lane highways only (including two-way frontage roads).

Recommended passing sight distances are based on the following assumptions:

In the design of two-lane highways, minimum or greater passing sight distance should be provided wherever practical, since less than minimum distances reduce the safety and LOS of the roadway. For rolling terrain, provision of climbing lanes may be a more economical alternative than achieving a vertical alignment with adequate passing sight distance.

The minimum passing sight distance for a two-lane road is about twice the minimum stopping sight distance at the same design speed. To meet those greater sight distances, clear sight areas on the inside of curves should be provided. For cut sections, designing for passing sight distance should be limited to tangents and very flat curves. Even in level terrain, providing passing sight distance would need a clear area inside each curve that would, in some instances, extend beyond the normal ROW line.

Minimum passing sight distance values for design of two-lane highways are shown in .

These distances are for design purposes only and should not be confused with other distances used as warrants for striping no-passing zones as shown in the

. For the design of typical two-lane rural highways, except for level terrain, provision of near continuous passing sight distance is impractical. However, the designer should attempt to increase the length and frequency of passing sections where economically feasible.

Intersection sight distance is fundamental to intersection operation and

should be calculated at every intersection

to ensure a safe and traversable intersection. Calculations and more information about intersection sight distance can be found in .

Sight distance through a grade crossing should be at least the minimum stopping sight distance, or longer. Line of sight may be obstructed by an overpass structure and can limit the sight distance for the operator. Where practical, provide the minimum length of sag vertical curve at grade separated structures. Where economically feasible, for two-lane roadways, the passing sight distance should be maintained.

When the minimum sag vertical curve length based on headlight sight distance, as described in , is achieved for vertical clearances of 14-ft or higher, then and subsequent equations are not applicable. However, if any of the following conditions occur, then the under-crossing curve length as shown in and subsequent equations should be verified based on the applicable stopping sight distance (SSD):

The general equations for sag vertical curve length at under-crossings are:

Case 1 –

Sight distance greater than length of vertical Curve (S>L):

$\text{L = 2S -}\frac{800\left[C-{\displaystyle \frac{h1+h2}{2}}\right]}{A}$

Where:

L =

length of vertical curve, ft

S =

sight distance, ft

C =

vertical clearance, ft

h

1

=

eye height, ft

h

2

=

height of object, ft

A =

algebraic difference in grades, percent

Case 2 –

Sight distance less than length of vertical curve (S<L)

$\text{L =}\frac{A{S}^2}{800\left[C-{\displaystyle \frac{h1+h2}{2}}\right]}$

Using an eye height of 8.0-ft for a truck driver and an object height of 2.0-ft for the taillights of a vehicle, the following equations can be derived:

Case 1 –

Sight distance greater than length of vertical curve (S>L):

$\text{L =2S -}\frac{800\left(C-5\right)}{A}$

Case 2 –

Sight distance less than length of vertical curve (S<L)

$\text{L =}\frac{A{S}^2}{800\left(C-5\right)}$

Where:

L =

length of vertical curve, ft

S =

sight distance, ft

C =

vertical clearance, ft

h

1

=

height of eye, ft

h

2

=

height of object, ft

A =

algebraic difference in grades, percent