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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
March 1, 1997
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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.

  1. 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.

  2. 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.

  3. 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.

  4. 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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