Civil Engineering Reference
In-Depth Information
delays or almost 300 million vehicle hours of delay on major roadways alone. Over
75 % of the signals in the US could be improved by adjusting and updating
equipment or by adjusting timing and coordination plans. The FHWA recommends
that jurisdictions assess retiming signals at least once every 3 years.
16.4.1 Signal Location and Spacing
Traf
c signals should be installed at intersections that meet the national Manual on
Uniform Traf
c Control Devices recommendations [
3
] and state standards per-
taining to minimum volume and annual crashes. Too many signals along a roadway
generally should be avoided, since adding signals generally reduces travel speeds
and increases delay (see Table
16.1
).
Typically signals are located almost every block in the central business district
and other major commercial centers. In cities, average spacing should not be closer
than one quarter mile; while wider signal spacing is desirable in suburban areas.
16.4.2 Cycle Length
The cycle lengths should (1) accommodate con
c volumes at intersec-
tions without excessive delay on each approach, (2) provide enough green time on
side streets to let pedestrians cross safely, and (3) permit effective coordination of
signals along a roadway and. Within this context, cycle lengths should be as short
as possible. When cycle lengths are too long, delays can be excessive. Although
cycle lengths depend upon the number of phases, they generally should not be
longer than 2 min. Shorter cycles are desirable where transit vehicles operate along
the streets.
fl
icting traf
Table 16.1 Percentage
increase in travel times and
increasing signal density
Signals per mile
Percent increase in travel times
(compared with 2 signals per mile)
2.0
0
3.0
9
4.0
16
5.0
23
6.0
29
7.0
34
8.0
39
Source
Reference [
4
]
