Ch9-4, pp.650-677: Intersection Sight
Distance
Learn the types of approach sight triangles
„ Determine proper sight distances for various
intersection traffic controls (including
uncontrolled intersections)
„ Determine the effect of skewed approach on
intersection sight distance
„
General considerations
The provision of SSD at all locations along each
highway or street, including intersection approaches, is
fundamental to intersection operation.
Sight distance is provided at intersections to allow
drivers to perceive the presence of potentially conflicting
vehicles.
The methods for determining the sight distances
needed by drivers approaching intersections are based on
the same principles as SSD, but incorporate modified
assumptions based on observed behavior at intersections.
Sight Triangles (p.651)
Two types of clear sight triangles are considered in
intersection design: approach sight triangles and
departure triangles.
Approach Sight Triangles
When the driver of a vehicle without the ROW sees a
vehicle that has the ROW on an intersecting approach, the
driver of that potentially conflicting vehicle can also see the
first vehicle. (This is a case of uncontrolled intersections.)
Departure Triangles
The departure sight triangle provides sight distance
sufficient for a stopped driver on a minor road
approach to depart from the intersection and enter or
cross the major road.
Identification of Sight Obstruction
within Sight Triangles
Consider both the horizontal and vertical alignment of both
intersecting roadways, as well as the height and position of the
object.
Assumptions:
Driver’s eye height = 3.5 ft (SU and combination trucks);
= 7.6 ft can be used for a truck driver’s eye height.
Object to be seen = 3.5 ft (= vehicle height 4.35 ft – 10
inch allowance). As it turns out this is reciprocal: the other
driver can see your vehicle.
Intersection Control
The recommended dimensions of the sight triangles vary with the type of
traffic control used at an intersection because different types of control
impose different legal constraints on drivers and, therefore, result in
different driver behavior.
Case A – Intersections with No Control
Concept: just like SSD with slightly different assumptions. Reaction time
is still 2.5 sec. But vehicles approaching uncontrolled intersections
typically slow to about 50% of their midblock running speed. This case
uses the approach sight triangle concept, for both a and b of approach
sight triangle.
Graphic version of Case A – No
Traffic Control
These values are shorter than SSDs in Exhibit 3-1. Remember it is assumed
that approach speed reduces to 50% of the midblock speed.
No departure sight triangle is needed. If approach sight triangle conditions
are met, this condition is met.
These are multiplication coefficients
for approach grades greater than 3%.
Case B – Intersections with Stop
Control on the Minor Road
„
Since vehicles from the minor road stop at
the stop bar, the departure sight triangle
needs to be used.
‹ Case B1 - Left turns from the minor road
‹ Case B2 – Right turns from the minor
road,
‹ Case B3 – Crossing the major road from
a minor-road approach
The location of the vertex (decision point) = 14.5 to 18 ft. When 18 ft is
assumed, vehicles stop at 10 ft from the edge of the major road and the
distance from the front of the vehicle to the driver’s eye is 8 ft.
To this value add 1/2 lane for vehicles approaching from the left, or 1.5 lane
width for vehicles approaching from the right. Î Distance “a” on the minor
road.
Distance “b” Î determined by the distance traveled by vehicles traveled on
the major road during the average critical gap selected by drivers on the minor
road.
Critical gaps used by drivers on the
minor road
These
default
values are
for 2-lane
2-way
highways.
See the examples on page 660. Pay attention to the adjustments for
grades greater than 3.0%.
This is for passenger cars. Trucks? Make
adjustments to critical gaps.
Here is a graphical presentation of
Case B1 – Left turn from stop
If the sight distance along the major road shown in Exhibit 9-55, including
any adjustments, cannot be provided, then consideration should be given to
installing regulatory speed signing on the major-road approaches.
Case B2 – Right Turn from the Minor Road
The same concept used for left turns is used with shorter
gap times (overall 1.0 sec shorter) as shown below.
Exhibit 9-58 shows the results.
Exhibit 9-59 shows PC, SU, Comb values
If this b-distance condition is not met, consideration of installing
regulatory speed signing or other traffic control devices on the
major-road approaches.
Case B3 – Crossing Maneuvers from
the Minor Road
„
In most cases, the departure sight triangles for left and
right turns onto the major road, as described fro Case B1
and B2, will also provide more than adequate sight
distance for minor-road vehicles to cross the major road.
„
For the following cases, check the available sight distance
‹
‹
‹
The crossing maneuver is the only legal maneuver,
Crossing more than six lanes, or
High volume of heavy vehicles and steep grades on the far side of
the intersection.
Case C – Intersections with Yield
control on the Minor Road
Drivers are permitted to enter or cross the major road
without stopping, if there are no potentially conflicting
vehicles on the major road. The sight distances needed by
drivers on yield-controlled approaches exceed those for
stop-controlled approaches.
For four-leg intersections with yield control on the minor
road, approach sight triangles for crossing and approach
sight triangles for left- and right-turns need to be checked.
Case C1 – Crossing Maneuver from the Minor
Road
Similar to Case A: no
control, but assumed to
reduce speed to 60%
(1.47*0.6=0.88) instead
of 50% for distance a.
Use Equation 9-2 to
estimate travel time to
reach and clear the
major road and to
compute distance b.
tg values used for Case C1-Yield, Crossing
Maneuvers (note distance “a” is given here)
Looks like w = 24 ft and La = 20 ft were used, though not
explained in the text.
Case C1 computed b values
Case C1 graphical presentation
Case C2 – Left and Right-Turn Maneuvers
The length of the leg of the approach
triangle along the minor road (distance a in
sight triangle) should be 82 ft. Slow down
to 10 mph.
„ The length of the leg of the major road
(distance b) is computed using time gap
values shown in the next slide.
„
Time gap used for Case C2
Distance “b” along the major road for Case
C2 (this uses approach triangle)
Graphical presentation of b for Case C2
Departure sight triangles are not checked because approach sight triangle
values are larger then them.
If these sight triangles are not available, consider a stop sign.
Case D – Intersections with Traffic Signal
Control
„
„
„
Left turning vehicles should have sufficient sight
distance to select gaps in on-coming traffic and
complete left turns (this left turn is different from
other left turns we discussed). Apart from these
conditions, no other approach or departure sight
triangles need to be checked.
If two-way flashing operation is used, Case B2
requirements must be applied.
For RTOR, Case B2 sight triangles for right turns
are applied
Case F – Left Turns form the Major Road
Sight distance design should be based on a left turn by a
stopped vehicle. The distance along the major road is the
distance traveled at the design speed of the major-road.
Time gaps in the following exhibit are used.
Distance “b” for Case F
If SSD is continuously provided along the major road and if sight distance
for Case B or Case C is provided, usually Case F may not be checked
because the former values are larger than the values shown in this exhibit.
Graphical presentation of Case F
Effect of Skew
„
„
„
When two highways intersect at an angle less than 60
degrees and when realignment to increase the angle of
intersection is not justified, some of the factors for
determination of intersection sight distance may need to be
adjusted.
Each of the clear sight triangles described for other cases
are applicable to oblique-angle intersections. The major
road width will change (shown as W2 in the exhibit shown
in the next slide, which is w/sin(θ)).
When Case B requirement is used, adjust tg by the
estimated number of lanes (since the oblique distance
(width) of the major road increases). For Case C1, adjust w
term (w/sin(θ)).
Effect of Skew: what changes is the width of
the major road and how much you have to
turn your head to see clearly the approaching
vehicles.