Chapter 4H. Traffic Control Signals for Freeway Entrance Ramps
Section 4H.01 Application
of Freeway Entrance Ramp Control Signals
Ramp control signals are traffic control signals that control the
flow of traffic entering the freeway facility.
Freeway entrance ramp control signals are sometimes
used if controlling traffic entering the freeway could reduce the
total expected delay to traffic in the freeway corridor, including
freeway ramps and local streets, and if at least one of the following
conditions is present:
- Congestion recurs on the freeway because traffic demand is
in excess of the capacity, or congestion recurs or a high frequency
of crashes exist at the freeway entrance because of inadequate
ramp merging area. A good indicator of recurring freeway congestion
is freeway operating speeds less than 80 km/h (50 mph) occurring
regularly for at least a half-hour period. Freeway operating speeds
less than 50 km/h (30 mph) for a half-hour period or more would
indicate severe congestion.
- Controlling traffic entering a freeway assists in meeting local
transportation system management objectives identified for freeway
traffic flow, such as the following:
- Maintenance of a specific freeway level of service.
- Priority treatments with higher levels of service for mass
transit and carpools.
- Redistribution of freeway access demand to other on-ramps.
- Predictable, sporadic congestion occurs on isolated sections
of freeway because of short-period peak traffic loads from special
events or from severe peak loads of recreational traffic.
The installation of ramp control signals should be preceded by an
engineering study of the physical and traffic conditions on the
highway facilities likely to be affected. The study should include
the ramps and ramp connections and the surface streets that would
be affected by the ramp control, as well as the freeway section
concerned. Types of traffic data that should be obtained include,
but are not limited to, traffic volumes, traffic crashes, freeway
operating speeds, and travel time and delay on the freeway, approaches,
ramps, and alternate surface routes.
Capacities and demand/capacity relationships should
be determined for each freeway section. The locations and causes
of capacity restrictions and those sections where demand exceeds
capacity should be identified. From these and other data, estimates
should be made of desirable metering rates, probable reductions
in the delay of freeway traffic, likely increases in delay to ramp
traffic, and the potential impact on surface streets. The study
should include an evaluation of the ramp’s storage capacities
for vehicles delayed at the signal, the impact of queued traffic
on the local street intersection, and the availability of suitable
alternate surface routes having adequate capacity to accommodate
any additional traffic volume.
Before installing ramp control signals, consideration
should be given to their potential acceptance by the public and
the requirements for enforcing ramp control, as well as alternate
means of increasing the capacity, reducing the demand, or improving
the characteristics of the freeway.
Section 4H.02 Design
of Freeway Entrance Ramp Control Signals
Ramp control signals shall meet all of the standard design specifications
for traffic control signals, except as noted herein:
- The signal face for freeway entrance ramp control
signals shall be either a two-lens signal face containing red
and green signal lenses or a three-lens signal face containing
red, yellow, and green signal lenses.
- A minimum of two signal faces per ramp shall
face entering traffic.
- Ramp control signal faces need not be illuminated
when not in use.
Ramp control signals shall be located and designed
to minimize their viewing by mainline freeway traffic.
The required signal faces, if located at the side of the ramp roadway,
may be mounted such that the height above the pavement grade at
the center of the ramp roadway to the bottom of the signal housing
of the lowest signal face is between 1.4 m (4.5 ft) and 1.8 m (6
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