Page 98 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 81

ability to see a 2.0-ft-high (0.61-m) object in the roadway without being blocked by
the pavement surface. The height of eye for the driver used in the calculations is 3.5 ft
(1.07 m).
The sight distance on a sag vertical curve is dependent on the driver’s being able to
see the pavement surface as illuminated by headlights at night. The height of the headlight
is assumed to be 2.0 ft (0.61 m), and the height of the object is 0.0. The upward diver-
gence angle of the headlight beam is assumed to be 1°.
Intersection Sight Distance. A motorist attempting to enter or cross a highway from a
stopped condition should be able to observe traffic at a distance that will allow safe
movement. In cases where traffic is intermittent or moderate in flow, the motorist will
wait to find an acceptable “gap.” The driver approaching the intersection on the through
road should have a clear view of the intersection including any vehicles stopped, waiting
to cross, or turning. The methods described in the following paragraphs produce distances
that provide sufficient sight distance for the stopped driver to make a safe crossing or
turning maneuver. If these distances cannot be obtained, the minimum sight distance pro-
vided should not be less than the stopping sight distance for the through roadway. This
would allow a driver on the through roadway adequate time to bring the vehicle to a stop
if the waiting vehicle started to cross the intersection and suddenly stopped or stalled. If
this distance cannot be provided, additional safety measures must be provided. These
could include, but are not limited to, advance warning signals and flashers and/or reduced
speed limit zones in the vicinity of the intersection.
There are three possible maneuvers for a motorist stopped at an intersection to
make. The motorist can (1) cross the intersection by clearing oncoming traffic on both
the left and right of the crossing vehicle, (2) turn left into the crossing roadway after
first clearing the traffic on the left and then making a safe entry into the traffic stream
from the right, or (3) turn right into the crossing roadway by making a safe entry into
the traffic stream from the left.
In order to evaluate the amount of sight distance available to a stopped vehicle
waiting to make a crossing or turning maneuver, the American Association of State
Highway and Transportation Officials (AASHTO) adopted the concept of using “sight
triangles” (Ref. 1). The vertices of the triangles are (a) the waiting driver’s position,
(b) the approaching driver’s position, and (c) the intersection of the paths of the two
vehicles, assuming a straight-ahead path for the waiting vehicles. Figure 2.7 shows the
concept of sight triangles, emphasizing both the horizontal and vertical elements to be
considered. The shaded area in the triangles is to be free of objects that would obstruct
the field of vision for either driver. The profile view shows the limiting effect of vertical
curvature of the through roadway. Notice that the height of eye of the drivers (3.50 ft or
1.07 m) is used for both the waiting and approaching vehicles. This stresses the
importance of both drivers being able to see each other.
Table 2.3A provides intersection sight distance values for through vehicle speeds
from 15 to 70 mi/h (24 to 113 km/h). The distances are based on a time gap of 7.5 s for
a passenger vehicle turning left and a gap of 6.5 s for a crossing or right-turning vehicle.
The height of eye and object were taken as 3.50 ft (1.07 m). The table also provides
K values for crest vertical curves that would provide the required sight distance. (See
Art. 2.2.4 for a discussion of vertical curvature.) Formulas are provided so that dis-
tances can be calculated for trucks requiring a longer time gap and for time adjust-
ments due to upgrades or multiple lane crossings. See the notes in Table 2.3A, which
explain how to adjust the timings.
Passing Sight Distance. In Table 2.3B, the “PSD” column lists the distances required
for passing an overtaken vehicle at various design speeds. These distances are applicable