Chapter 4I. Traffic Control for Movable Bridges
Traffic control signals for movable bridges are a special type of highway traffic signal installed at movable bridges to notify road users to stop because of a road closure rather than alternately giving the right-of-way to conflicting traffic movements. The signals are operated in coordination with the opening and closing of the movable bridge, and with the operation of movable bridge warning and resistance gates, or other devices and features used to warn, control, and stop traffic.
Movable bridge warning gates installed at movable bridges decrease the likelihood of vehicles and pedestrians passing the stop line and entering an area where potential hazards exist because of bridge operations.
A movable bridge resistance gate is sometimes used at movable bridges and located downstream of the movable bridge warning gate. A movable bridge resistance gate provides a physical deterrent to road users when placed in the appropriate position. The movable bridge resistance gates are considered a design feature and not a traffic control device; requirements for them are contained in AASHTO’s "Standard Specifications for Movable Highway Bridges" (see Addresses for AASHTO’s address).
Traffic control at movable bridges shall include both signals and gates, except in the following cases:
- Neither is required if other traffic control devices or measures
considered appropriate are used under either of the following
- On low-volume roads (roads of less than 400 vehicles average daily traffic); or
- At manually operated bridges if electric power is not available.
- Only signals are required in urban areas if intersecting streets or driveways make gates ineffective.
- Only movable bridge warning gates are required if a traffic control signal that is controlled as part of the bridge operations exists within 150 m (500 ft) of the movable bridge resistance gates and no intervening traffic entrances exist.
The signal heads and mountings of movable bridge signals shall follow the provisions of Chapter 4D except as noted in this Section.
Since movable bridge operations cover a variable range of time periods between openings, the signal faces shall be one of the following types:
- Three-section signal faces with red, yellow, and green signal lenses; or
- Two one-section signal faces with red signal lenses in a vertical array separated by a STOP HERE ON RED (R10-6) sign (see Section 2B.45).
Regardless of which signal type is selected, two signal faces shall be provided for each approach to the movable span.
If movable bridge operation is frequent, the use of three-section signal faces should be considered.
If physical conditions prevent a road user from having a continuous view of at least two signal indications for the distance specified in Table 4D-1, an auxiliary device (either a supplemental signal face or the mandatory DRAWBRIDGE AHEAD warning sign to which has been added a warning beacon that is interconnected with the movable bridge controller unit) shall be provided in advance of movable bridge signals and gates.
A DRAWBRIDGE AHEAD warning sign shall be used in advance of movable bridge signals and gates to give warning to road users, except in urban conditions where such signing would not be practical.
Movable bridge warning gates, if used, shall extend at least across the full width of the approach lanes if movable bridge resistance gates are used. On divided highways in which the roadways are separated by a barrier median, movable bridge warning gates, if used, shall extend across all roadway lanes approaching the span openings. Except where physical conditions make it impractical, movable bridge warning gates shall be located 30 m (100 ft) or more from the movable bridge resistance gates or, if movable bridge resistance gates are not used, 30 m (100 ft) or more from the movable span.
Movable bridge warning gates shall be at least standard railroad size, striped with 400 mm (16 in) alternate diagonal, fully reflectorized red and white stripes. Flashing red lights in accordance with the Standards for those on railroad gates (see Section 8D.04) shall be included on the gate arm and they shall only be operated if the gate is closed or in the process of being opened or closed. In the horizontal position, the top of the gate shall be approximately 1.2 m (4 ft) above the pavement.
If two sets of gates (both a warning and a resistance gate) are used for a single direction, highway traffic signals need not accompany the resistance gate nearest the span opening, but there shall be flashing red lights on the movable bridge warning gate.
Signal faces with 300 mm (12 in) diameter signal lenses should be used for movable bridge signals.
Insofar as practical, the height and lateral placement of signal faces should conform to the requirements for other traffic control signals in accordance with Section 4D.15. They should be located not more than 15 m (50 ft) in advance of the movable bridge warning gate.
Movable bridge warning gates should be of lightweight construction. In its normal upright position, the gate arm should provide adequate lateral clearance. If the movable bridge is close to a highway-rail grade crossing and traffic might possibly be stopped on the crossing as a result of the bridge opening, a traffic control device should notify the road users to not stop on the railroad tracks.
If movable bridge resistance gates are not used on undivided highways, movable bridge warning gates, if used, should extend across the full width of the roadway.
On bridges or causeways that cross a long reach of water and that might be hit by large marine vessels, within the limits of practicality, traffic should not be halted on a section of the bridge or causeway that is subject to impact.
In cases where it is not practical to halt traffic on a span that is not subject to impact, traffic should be halted at least one span from the opening. If traffic is halted by signals and gates more than 100 m (330 ft) from the movable bridge warning gates (or from the span opening if movable bridge warning gates are not used), a second set of gates should be installed approximately 30 m (100 ft) from the gate or span opening.
Movable bridge signals may be supplemented with audible warning devices to provide additional warning to drivers and pedestrians.
If prevailing approach speeds are 40 km/h (25 mph) or less, signal heads with 200 mm (8 in) diameter lenses may be used.
The movable bridge resistance gates may be delineated, if practical, in a manner similar to the movable bridge warning gate.
The DRAWBRIDGE AHEAD sign may be supplemented by a Warning Beacon (see Section 4K.03).
A single full-width gate or two half-width gates may be used.
Highway traffic signals need not accompany the gates nearest the span opening.
The locations of movable bridge signals and gates are determined by the location of the movable bridge resistance gate (if used) rather than by the location of the movable spans. The movable bridge resistance gates for high-speed highways are preferably located 15 m (50 ft) or more from the span opening except for bascule and lift bridges, where they are often attached to, or are a part of, the structure.
Traffic control devices at movable bridges shall be coordinated with the movable spans, so that the signals, gates, and movable spans are controlled by the bridge tender through an interlocked control.
If the three-section type of signal face is used, the green signal indication shall be illuminated at all times between bridge openings, except that if the bridge is not expected to open during continuous periods in excess of 5 hours, a flashing yellow signal indication may be used. The signal shall display a steady red signal indication when traffic is required to stop. The duration of the yellow change interval between the display of the steady green and steady red signal indications, or flashing yellow and steady red signal indications, shall be predetermined.
If the vertical array of red signal lenses is the type of signal face selected, the red signal lenses shall flash alternately only when traffic is required to stop.
The duration of the yellow change interval should have a range from 3 to 6 seconds.
Signals on adjacent streets and highways should be interconnected with the drawbridge control if indicated by engineering judgment.