Wyoming Adopts New Breakaway Gate for Winter Weather Road Closures
When the Wyoming Department of Transportation (WYDOT) requested
evaluation of a new road closure gate, the Texas Transportation Institute (TTI) crash
tested WYDOT's proposed gate design, modified the design, and crash tested the
revised design. The modified design gained Federal Highway Administration (FHWA)
approval. King K. Mak, Roger P. Bligh, and William B. Wilson described the new gate
and the testing process in their paper called "Wyoming Road Closure Gate." The paper
was presented at the Seventy-Fifth Annual Meeting of the Transportation Research
Board in Washington, D.C. This article describes some of their findings.
BACKGROUND
If driving becomes extremely hazardous due to severe winter weather, WYDOT
closes affected State highways. For years, Wyoming and other states used swing gates
positioned at the outskirts of most cities to close these highways to traffic. The swing
gates, however, were difficult for field personnel to close under the conditions that
required their use--high winds and blowing snow. Closing gates was more difficult when
cables used to anchor the gates in the closed position occasionally became tangled
during high winds. Moreover, to close swing gates, employees must walk onto the
roadway and place themselves at high risk, especially when roads are icy and visibility is
poor. Finally, swing gates required a lot of maintenance and were not regarded as
crashworthy if hit by a vehicle.
Wanting to improve their method of closing winter roads, WYDOT put together a
committee composed of members of the Highway Patrol and personnel representing the
areas of highway design, construction, and maintenance. Highway Patrol advised it
would be extremely difficult to enforce highway closures without using physical barriers.
The committee, therefore, explored alternate gate designs.
With consideration for safety and cost, the committee developed a design that
used a railroad arm installed on a light pole with a breakaway base. This design would
keep highway personnel off the road. In addition, because the new gate used parts
already stockpiled in highway maintenance yards, production could begin quickly and
with minimum expense.
WYDOT then asked the Texas Transportation Institute (TTI) to evaluate the
committee's gate design. Mak, Bligh, and Wilson described the evaluation:
The objectives of this study were to crash test and evaluate the new Wyoming
road closure gate design to determine if the design would meet the appropriate
impact performance guidelines and specifications and to improve the design from
the standpoints of safety performance, cost and practicality. The scope of the
study included engineering analysis of the existing road closure gate design,
followed by full-scale crash testing and evaluation of the design.
The final gate needed to meet the performance criteria in National Cooperative
Highway Research Program (NCHRP) Report 350 and the American Association of
State Highway and Transportation Officials' (AASHTO's) Standard Specifications for
Structural Supports for Highway Signs, Luminaires, and Traffic Signals.
GATE DESIGN
The road closure gate that performed successfully in crash tests and met NCHRP
and AASHTO standards has four major components: support pole structure, breakaway
mechanism, gate arm, and gate arm attachment with lift mechanisms. Below are brief
descriptions of each component.
Support Pole Structure
The support pole is a standard, 29-foot roadway light pole with an 8-foot mast arm
and an attached streetlight. The pole tapers from an outside diameter of 8 inches at the
base to 4 inches at the top. Including the base assembly, the total structure height is 29
feet 4 inches.
Breakaway Mechanism
The four-bolt slip base design used for the final snow-gate assembly had
previously met national standards for use with a streetlight structure. The 900-pound
assembly bolts to a concrete foundation.
Gate Arm
The snow gate uses a commercially available fiberglass/aluminum gate arm. The
12-foot-long aluminum base can be extended with a telescoping fiberglass section.
Maximum recommended length is 32 feet. Breakaway bolts attach the support gate arm
to the support pole structure. Designed to fail when a vehicle hits the arm, these bolts
allow the arm to rotate around a pivot rod. This prevents damage to the vehicle and the
gate. In addition, red-and-white-striped retroreflective sheeting covers the arm, and three
red-lensed lamps on the arm further improve visibility.
Gate Arm Attachment and Lift Mechanisms
The gate arm attaches to the pole with a steel pivot rod. Special bearings and
housings make the design uniquely suited to operation under adverse weather
conditions. The electric jack mechanism takes about two-and-one-half minutes to raise
the gate arm. A special gate-arm bracket restricts lateral movement of the gate when it is
in the up position.
COMPLIANCE TESTING
TTI did three full-scale crash tests during their study of the Wyoming snow gate:
two low-speed (22-miles-per-hour) tests and one high-speed (62-miles-per-hour) test.
The objective of low-speed tests was to evaluate the breakaway mechanism. The high-
speed test aimed to assess the trajectory of the vehicle and snow gate components after
impact.
The first low-speed test used the committee's original design. This road closure
gate design failed to meet the evaluation criteria. Upon impact, the support pole
structure rotated free and hit the rear of the vehicle roof, breaking the glass and severely
damaging the roof. The test "was considered a failure due to intrusion into the occupant
compartment resulting from the secondary impact of the separated support pole structure
with the roof of the test vehicle."
WYDOT considered several redesign options. They selected the option of
lengthening the pole structure. The original design used an 18-foot pole. The final
design used a 29-foot pole. The redesign also incorporated a mast arm and light. The
taller pole and the addition of the mast arm and lighting fixture were the only changes
made in the original committee design.
The second low-speed crash test used the modified snow gate design. The taller
pole equipped with a mast arm and light touched the rear of the test vehicle only briefly.
There was no apparent intrusion into the passenger section of the vehicle; the redesign
had reduced the risk to occupants of a vehicle hitting the gate. This design produced
test results well within the recommended limits of NCHRP Report 350.
TTI used the redesigned gate assembly for a high-speed crash test. The test
vehicle received moderate damage at the point of impact--the right front. The support
pole did not contact the vehicle in any other location. While damage to the vehicle was
small, damage to the gate was extensive. The entire installation required replacement.
However, "all occupant risk factors were well within the acceptable limits set forth in
NCHRP Report 350."
DISCUSSION
The authors concluded that the redesigned road closure gate successfully met
guidelines established by NCHRP and AASHTO. WYDOT adopted the modified gate
design and the Federal Highway Administration accepted it for use on the National
Highway System.
The authors submitted four points for further consideration.
- If a highway department were to use the snow gate without equipping the
pole with a mast arm and streetlight, the assembly would require a taller
pole.
- The gate-arm bracket supports the gate arm when it is in the up position
and restricts its lateral movement. For the test, the gate-arm bracket was
18 feet above the base of the support pole unit. While this height will
change with the length of the gate arm, the authors recommended a
minimum mounting height of 18 feet.
- The road closure gate assembly used in these crash tests had a four-bolt,
slip-base breakaway mechanism. The authors felt other crash-tested and
approved breakaway bases (e.g., breakable couplings, a breakable
transformer base, or a three-bolt slip base) would also perform
satisfactorily.
- The crash-tested gate design used an electric in-line linear actuator to lift
the gate arm mechanism. A manual winch and pulley or other alternate lift
mechanism could be used and would not negatively affect the gate's
performance. The authors cautioned that this would be true only if the
mechanism chosen did not significantly increase the weight or size of the
unit.
Copyright © 1997 by TranSafety, Inc.
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