Civil Engineering Reference
In-Depth Information
Table 16.4 Estimated time lost per cycle by con
fl
icting right turns and pedestrian volumes
Typical values of
R/Nc
and
P/Nc
Time loss per cycle at 3 s per pedestrian channels
(lanes)
1 Lane
2 Lanes
3 Lanes
4 Lanes
4
12
6
4
3
8
24
12
8
6
12
36
18
12
9
16
48
24
16
12
20
60
30
20
15
72
a
36 24 18
Notes
For a 60 s cycle, time loss should not exceed 25 % of cycle or 15 s. Thus, values below the
boldface lines are not acceptable, and turns should be prohibited
Source
Reference [
17
]
R
= right turns per hour
Nc
= number of cycles per hour
P
= pedestrians per hour
a
24
= Excess cycle length
Therefore, if right turns were prohibited in this case the curb lane would then
gain an additional
20 s of additional green time per signal cycle.
Sometimes it is possible to provide curb lanes for moving traf
c by removing
curb parking, by restriping the roadway, or by minor widening.
16.6.2 Managing Left Turns
Delays from left turn movements (for right-hand driving) are common at inter-
sections along most streets and roads. Depending upon speci
c circumstances,
delays created by left turns generally can be reduced by prohibiting, better
accommodating, diverting, or separating left turn movements. Table
16.5
summa-
rizes the various ways of addressing the left turn issue and suggests where each
possible improvement applies [
5
].
16.6.3 Left-Turn Treatment Options
This chapter describes how to minimize the left-turn con
icts at intersections
through operational changes. While the next chapter describes how left turn con-
fl
fl
flicts can be separated through the redesign of the intersection.
(a) Prohibiting Left Turns at Intersections
The prohibition of left turns is common at heavily traveled intersections,
especially where it is not possible to provide protected left turn lanes. The
