wyoming road closure gate - transportation research...

Road closure gates are used to close certain highways when driving con-ditions become too hazardous under severe winter weather conditions.The Wyoming Department of Transportation (WYDOT) developed anew road closure gate design that had not been crash tested to determinewhether it would meet nationally recognized safety standards. WYDOTsponsored a study at the Texas Transportation Institute to crash test andevaluate the new road closure gate design and, as appropriate, toimprove the design from the standpoints of safety performance, cost,and practicality. The original road closure gate design was crash testedand failed to meet the guidelines set forth in NCHRP Report 350 and the1985 AASHTO Standard Specifications for Structural Supports forHighway Signs, Luminaires, and Traffic Signals. The design was thenmodified and crash tested with successful results. The modified road clo-sure gate design consists of a standard 8.84-m (29-ft)-high luminairesupport pole structure with a mast arm and light standard, a four-bolt slipbase breakaway base, a telescoping fiberglass-aluminum gate arm withan electric in-line linear actuator lift mechanism, and a gate arm bracketto restrict the lateral movement of the gate arm when it is in the up posi-tion. The road closure gate design has been adopted by WYDOT andaccepted by FHWA for use on the National Highway System.

The Wyoming Department of Transportation (WYDOT) uses roadclosure gates to close certain highways when driving conditionsbecome too hazardous under severe winter weather conditions.Gates typically are located at the outskirts of most cities on thestate’s highway system. WYDOT and other states have used a swinggate design for years. However, many problems were identified withthe swing gate design. The swing gates were difficult for field per-sonnel to close under the conditions of high winds and blowingsnow normally associated with the need to close these highways.Field personnel had to walk out onto the roadway to close thesegates, placing themselves at high risk under low visibility and icyroad conditions. The swing gates had cables to anchor the gates inthe closed position, but these would occasionally become tangledduring windy conditions. Furthermore, the swing gates requireextensive maintenance to keep them operational. Also, the swinggates were not believed to be crashworthy if they were struck.

WYDOT formed a committee consisting of patrol, maintenance,construction, and design personnel to develop an improved methodof road closures. The Highway Patrol indicated that it would be verydifficult to enforce road closures without a physical barrier in place.The committee explored various ideas and eventually arrived at adesign that would incorporate a railroad arm mounted on a luminairepole with a breakaway base. It was believed that this would be amuch safer design than the existing swing gates, both for the fieldpersonnel and for the traveling public. Also, many of the compo-nents required for the new gate design already existed in mainte-nance stockpiles, so that replacement of the gates with gates with the

38 TRANSPORTATION RESEARCH RECORD 1528

new design could proceed promptly without WYDOT’s incurringmajor expenses.

The road closure gate design developed by the committee had notbeen crash tested to determine whether it would meet nationally rec-ognized safety standards, that is, the performance criteria outlinedin NCHRP Report 350 (1) and the 1985 AASHTO Standard Speci-fications for Structural Supports for Highway Signs, Luminaires,and Traffic Signals (2). WYDOT therefore initiated a study with theTexas Transportation Institute to evaluate the gate design, to makerecommendations for improvements, and to conduct full-scale crashtesting to ensure that the road closure gate design would meetnationally recognized safety standards.

The objectives of the study were to crash test and evaluate the newWyoming road closure gate design to determine whether the designwould meet the appropriate impact performance guidelines and spec-ifications and to improve the design from the standpoints of safetyperformance, cost, and practicality. The scope of the study includedengineering analysis of the existing road closure gate design, fol-lowed by full-scale crash testing and evaluation of the design.

ROAD CLOSURE GATE DESIGN

A schematic of the modified road closure gate design that was suc-cessfully crash tested is shown in Figure 1. Photographs of the testinstallation are provided in Figure 2. The major components of thedesign are

• Support pole structure,• Breakaway mechanism,• Gate arm, and• Gate arm attachment and lift mechanisms.

The following sections provide brief descriptions of these com-ponents.

Support Pole Structure

A standard 8.84-m (29-ft)-high luminaire support pole structure,with a 2.44-m (8-ft)-long mast arm and light standard, is used withthe road closure gate, a schematic of which is shown in Figure 1.The pole shaft and mast arm shaft are made from 11-gauge hot-rolled ASTM A595 Grade A carbon steel and are hot-dip galva-nized in accordance with ASTM Standard A123. The pole shaft hasan outside diameter of 203 mm (8 in.) at the base and 102 mm (4in.) at the top, with a linear taper rate of 1.2 mm/m (0.14 in./ft). Thestub height of the permanent lower slip base assembly is 102 mm(4 in.), making the height to the top of the support pole structure8.94 m (29 ft 4 in.). The approximate weights of the componentsare 109 kg (240 lb) for the pole shaft including the top slip baseplate, 20 kg (44 lb) for the mast arm, and 18 kg (40 lb) for the lightstandard assembly.

Wyoming Road Closure Gate

KING K. MAK, ROGER P. BLIGH, AND WILLIAM B. WILSON

K. K. Mak, Safety Division, Texas Transportation Institute, Texas A&MUniversity, College Station, Tex. 77843-3135. R. P. Bligh, Structures Divi-sion, Texas Transportation Institute, Texas A&M University, College Sta-tion, Tex. 77843-3135. W. B. Wilson, Wyoming Department of Transporta-tion, P.O. Box 1708, 5300 Bishop Boulevard, Cheyenne, Wyo. 82002-9019.

Mak et al. 39

Breakaway Mechanism

A four-bolt slip base design, shown in Figure 3, is used with the roadclosure gate. This four-bolt slip base design was successfully crashtested previously with a 15.2-m (50-ft) luminaire support weighingapproximately 408 kg (900 lb) (3). In actual field installations thepermanent lower slip base assembly is bolted to a concrete founda-tion with four 25.4-mm (1-in.)-diameter AASHTO M314 Grade 55anchor bolts on a 406-mm (16-in.)-diameter bolt circle. However,for the test installation, the base assembly was bolted to an existinguniversal steel mounting plate with four 25.4-mm (1-in.)-diameterASTM A325 bolts. The permanent lower slip base assembly has astub height of 102 mm (4 in.). The top and bottom base plates arefastened together with four 25.4-mm (1-in.)-diameter ASTM A325slip bolts on a 330-mm (13-in.)-diameter bolt circle and 76.2-mm 3

FIGURE 1 Road closure gate design.

FIGURE 2 Road closure gate.

50.8-mm 3 12.7-mm (3-in. 3 2-in. 3 0.5-in.) plate washers. Theslip bolts are held in place with a 28-gauge keeper plate fabricatedfrom ASTM A526 material. The slip bolts are first tightened to atorque of 5.5 N-m (80 ft-lb) and are then released and retightened toa torque of 4.8 N-m (70 ft-lb), which is estimated to develop approx-imately 19.2 kN (4,300 lb) of tension per bolt (3).

Gate Arm

The gate arm used in the test installation was a commercially avail-able fiberglass-aluminum gate arm consisting of a 3.7-m (12-ft)-long base section of rectangular extruded aluminum and a secondtelescoping section made of pultruded fiberglass. The maximumrecommended length of the gate arm is 9.8 m (32 ft). The length ofthe gate arm used with the test installation was 7.3 m (24 ft). Thegate arm is attached to the support pole structure with a cast alu-minum breakaway mounting adapter that uses three 7.9-mm (0.3-in.)-diameter brass bolts. These bolts are designed to fail when thegate arm is hit by a vehicle, which allows the arm to rotate around a pivot rod, thus preventing major damage to the arm or to thevehicle striking the arm. The arm is covered with retroreflective


sheeting in red and white stripes and has three red-lensed lamps toprovide better visibility.

Gate Arm Attachment and Lift Mechanisms

The schematics and details of the gate arm attachment and lift mech-anisms are shown in Figure 4. The gate arm assembly is attached tothe support pole structure through a 38.1-mm (1.5-in.)-diameterASTM A36 steel pivot rod. Two gate arm plates are mounted ontothe pivot rod with two-bolt flange-mounted sleeve bearings. Thesesleeve bearings have Teflon-coated housings and chemical- and corrosion-resistant self-lubricating polymer sleeve inserts and aredesigned for operation in adverse environments.

Two channel spacers, one 381 mm (15 in.) long for attachment ofthe gate arm and the other 127 mm (5 in.) long for the upper con-nection of the electric in-line linear actuator (jack), are mountedbetween the gate arm plates. The electric jack is mounted onto thesupport pole structure with a bottom connection assembly. The elec-tric jack has a 3,400 rpm, 110-V alternating current motor with amaximum stroke of 460 mm (18.13 in.) and a maximum load capac-ity of 6.7 kN (1,500 lb). Testing with the actual installation showed

FIGURE 3 Schematic of four-bolt slip base breakaway design: (a) breakaway base, (b) slip base assembly, (c) top plate.

(a)

(b) (c)

that it takes approximately 2.5 min to raise the gate arm from thedown to the up position.

A gate arm bracket, shown in Figure 5, is attached to the sup-port pole structure to restrict the lateral movement of the gate arm inthe up position. The bracket for the test installation was mounted ata height of 5.5 m (18 ft) above the ground. However, this mountingheight of the bracket may vary depending on the length of the gatearm. Also, to avoid interference with the luminaire mast arm whenthe gate arm is in the up position, the mast arm is offset at an angleof 25 degrees, as shown in Figure 5.

Original Road Closure Gate Design

The road closure gate design described above is the design that wassuccessfully crash tested. WYDOT’s original road closure gatedesign was crash tested first. It failed to meet the guidelines set forthin NCHRP Report 350 (1) and the 1985 AASHTO Standard Speci-fications for Structural Supports for Highway Signs, Luminaires,and Traffic Signals (2). The original design was then modified andwas subsequently crash tested with successful results.

The original design was similar to the final design except for theheight of the pole structure and the absence of a mast arm and lightstandard. The pole support structure in the original design was 5.5 m(18 ft) high and had an outside diameter of 229 mm (9 in.) at the baseand 165 mm (6.5 in.) at the top. The weight of this 5.5-m (18-ft) sup-port pole and attachments, including the top slip base plate, wasapproximately 91 kg (200 lb). The other design details, including the

Mak et al. 41

breakaway mechanism, the gate arm, and the gate arm attachmentand lift mechanisms, were identical to those of the final design.

COMPLIANCE TESTING

Two compliance crash tests are required to evaluate the performanceof a breakaway support structure in accordance with guidelines setforth in NCHRP Report 350 (1) and the 1985 AASHTO StandardSpecifications for Structural Supports for Highway Signs, Lumi-naires, and Traffic Signals (2), as follows:

1. Test Designation 3-60. An 820-kg (1,808-lb) passenger carstriking the support pole structure head-on at a nominal impactspeed of 35 km/hr (21.8 mph). The objective of this test is to evalu-ate the breakaway mechanism of the support structure.

2. Test Designation 3-61. An 820-kg (1,808-lb) passenger carstriking the support pole structure head-on at a nominal impactspeed of 100 km/hr (62.2 mph). The objective of this test is to eval-uate vehicle and test article trajectory.

A total of three full-scale crash tests were conducted in the pres-ent study. The first test was a low-speed test (Test 3-60) with theoriginal road closure gate design. This test failed to meet the evalu-ation criteria because of an intrusion into the occupant compartment.After modifications to the support pole structure design, the low-speed crash test was rerun with successful results. The high-speedtest (Test 3-61) was also conducted with the modified road closuregate design with satisfactory results.

FIGURE 4 Gate arm attachment and lift mechanisms: (a) arm down, (b) arm up.

(a) (b)

The crash tests were conducted with the gate arm in the down position. This was considered a more critical test condi-tion since the gate arm might affect the trajectory of the supportpole structure after separating from the slip base assembly. FHWAwas consulted and agreed with this test configuration. Brief


descriptions of these three crash tests are presented in the follow-ing sections.

Low-Speed Crash Test

The first low-speed crash test was conducted on the original roadclosure gate design. As mentioned previously, the only differencesbetween the original road closure gate design and the final designwere the size and height of the support pole structure. The test instal-lation of the original road closure gate design is shown in Figure 6.

This test involved an 820-kg (1,808-lb) passenger car striking thegate head-on at a speed of 34.7 km/hr (21.6 mph), with the left frontquarter point of the vehicle aligned with the centerline of the sup-port pole structure. The test weight of the vehicle was 896 kg (1,975lb), including a restrained 50th percentile anthropomorphic dummyplaced in the driver’s position.

On impact the slip base activated as designed and the support polestructure rotated free of the slip base assembly. As the vehicle con-tinued traveling forward, the support pole structure rotated past the

FIGURE 5 Gate arm bracket.

FIGURE 6 Test installation of existing road closure gate design.

horizontal position and struck the rear of the roof of the vehicle,breaking the glass in the rear hatchback and damaging the roofseverely. The vehicle lost contact with the road closure gate travel-ing at 28.2 km/hr (17.5 mph). Sequential photographs of the test areprovided in Figure 7.

Damage to the road closure gate was minimal. The cast aluminumhousing of the electric motor was fractured during the test and wouldrequire repair or replacement before the road closure gate could bereinstalled. Although the front of the test vehicle sustained only

Mak et al. 43

minor damage, the roof of the vehicle was severely crushed. Themaximum recorded roof deformation was 160 mm (6.3 in.) acrossthe passenger compartment area and around the head of the anthro-pomorphic dummy that was positioned in the driver’s seat. Thedamage to the test installation and to the test vehicle is shown in Figures 8 and 9, respectively.

The occupant risk factors were well within the limits set forth in NCHRP Report 350 (1), as summarized in Table 1. However, this test was considered a failure because of the intrusion into the

0.000 s

0.128 s

0.257 s

0.385 s

FIGURE 7 Sequential photographs of first low-speed crash test.

occupant compartment resulting from the secondary impact of theseparated support pole structure on the roof of the test vehicle.Analysis of the high-speed film indicated that the separated supportpole structure had an angular velocity of approximately 180degrees/sec and was rotating about a point approximately 610 mm(2 ft) above the roof of the test vehicle. In comparison, a review ofhigh-speed film from successful crash tests with slip base luminairesupports indicated a much smaller angular velocity and higher pointof rotation for the separated support pole structure.

An engineering analysis based on conservation of linear andangular momentum principles was used to study the impact


performance and postimpact trajectory of the road closure gate sys-tem. On the basis of the results of the analysis, several options were explored with WYDOT, including lengthening or shorteningthe pole structure. The option of shortening the pole structure was rejected because of concern that (a) the shorter pole struc-ture may have the propensity to rotate into the windshield area of the vehicle striking the pole, which could result in intru-sion into the occupant compartment, and (b) the shorter polestructure would not provide any support for the long gate arm,which could result in damage to the gate arm under high windconditions.

0.513 s

0.641 s

0.770 s

0.898 s

FIGURE 7 Continued.

The option of lengthening the pole structure was thereforeselected. The basic concept behind this option was to reduce theangular velocity and raise the point of rotation of the separated sup-port pole structure by increasing the mass moment of inertia andcenter-of-gravity height, respectively. To accomplish this severalalternatives were evaluated, including different pole structurelengths and mast arm lengths.

After careful consideration it was decided that the 5.5-m (18-ft)-high pole structure would be replaced with a standard 8.84-m (29-ft)-high luminaire support to increase the mass moment of inertiaand the center-of-gravity height of the separated pole structure. Inaddition, a 2.44-m (8-ft)-long mast arm and a light standard wereincorporated into the design to further increase these properties andto provide better visibility. The taller support pole structure, withluminaire arm and luminaire, increased the weight of the road clo-sure gate system by 54 kg (118 lb), from 198 kg (437 lb) to 252 kg(555 lb). The mass moment of inertia was increased by a factor ofmore than five, from 1.648 3 109 mm4 (3,960 in.4) to 8.460 3 109

mm4 (20,326 in.4), and the height of the center of gravity was raisedfrom 2096 mm (82.5 in.) to 3653 mm (143.8 in.).

For the impact speed at which the first low-speed test was con-ducted, the predicted angular velocity of the separated pole structurewas reduced from 180 to 95 degrees/sec and the estimated rotationwould be less than 90 degrees before recontacting the vehicle or theground (i.e., the separated pole structure would not reach a hori-zontal position above the vehicle). Although the predicted velocitychange from impact with the modified road closure gate increasedfrom 1.7 to 2.1 m/sec (5.5 to 7 ft/sec) because of the additional

Mak et al. 45

FIGURE 8 Damage sustained by installation in first low-speedcrash test.

FIGURE 9 Damage sustained by vehicle in first low-speed crash test.

TABLE 1 Crash Test Results


weight, this value is still well below the acceptable limit of 5 m/sec(16.4 ft/sec).

Second Low-Speed Crash Test

The low-speed crash test was repeated with the taller, modifiedsupport structure. The test installation is shown in Figure 2. Thevehicle struck the modified road closure gate head-on at a speed of

31.8 km/hr (19.7 mph), with the left front quarter point of the vehi-cle aligned with the centerline of the support pole. On impact theroad closure gate released from the slip base as designed andachieved an angular velocity of approximately 73 degrees/sec. Asthe vehicle continued traveling forward, the support pole structurebriefly contacted the left rear corner of the roof of the vehicle. Thevehicle lost contact with the road closure gate traveling at 20.8km/hr (12.9 mph). Sequential photographs of the test are providedin Figure 10.

0.000 s

0.307 s

0.613 s

0.920 s

FIGURE 10 Sequential photographs of second low-speed crash test.

Damage to the road closure gate installation was relatively minor.The electric motor housing again was fractured and would requirerepair or replacement before reinstallation. Additionally, the lumi-naire was broken and would require replacement. The left front ofthe vehicle sustained light damage, as did the left rear of the roof.The roof was deformed downward 10 mm (0.4 in.) at the left rearcorner, and there was no apparent intrusion into the passenger com-partment area. The postimpact damage sustained by the vehicle andthe road closure gate is shown in Figure 11.

The occupant risk factors, summarized in Table 1, were wellwithin the recommended limits set forth in NCHRP Report 350 (1).

Mak et al. 47

The impact speed of 31.8 km/hr (19.7 mph) was below the targetimpact speed of 35 km/hr (21.7 mph). However, since a lowerimpact speed is generally considered to be more critical from thestandpoint of activating the break-away mechanism, the impactspeed is not considered to be a problem.

High-Speed Crash Test

The vehicle used in the low-speed crash test was reused for the high-speed crash test. The vehicle struck the road closure gate at an

1.227 s

1.533 s

1.840 s

2.146 s

FIGURE 10 Continued.

impact speed of 104.0 km/hr (64.6 mph), with the right front quar-ter point of the vehicle aligned with the centerline of the supportpole structure. On impact the slip base activated as designed, allow-ing the support pole structure to rotate freely above the vehicle. Thesupport pole eventually came to rest on the ground without contact-ing the vehicle. Sequential photographs of the test are provided inFigure 12.

The test installation and the vehicle after the test are shown inFigure 13. The road closure gate received extensive damage, and theentire installation would need to be replaced. The vehicle sustainedmoderate damage to the right front. The maximum crush at the rightquarter point of the bumper was 180 mm (7.1 in.).


As summarized in Table 1, all occupant risk factors were well within the acceptable limits set forth in NCHRP Report 350 (1). A small hole, measuring 10 mm (0.4 in.) 3 20 mm (0.8 in.), was punched in the passenger-side floor pan, and the oil pan was also punctured. On further investigation it appears that one of the slip bolts was first caught between the permanentlower slip base assembly and the undercarriage of the vehicle and then between the concrete pavement and the undercarriage of the vehicle, resulting in the damage. This behavior was indi-cated by gouge marks present on the top surface of the spacer plateof the permanent lower slip base assembly and on the concretepavement.

FIGURE 11 Damage sustained by test installation and vehicle in second low-speed crash test.

FIGURE 12 Sequential photographs of high-speed crash test.

0.000 s

0.086 s

FIGURE 12 (continued on next page)

0.172 s

0.258 s

0.344 s

0.430 s

On the basis of the results of the crash tests and analytic studies,the following considerations regarding the field implementation ofthe road closure gate are offered.

• A standard 8.8-m (29-ft)-high luminaire support pole structurewith an outside base diameter of 203 mm (8 in.) and equipped witha 2.4-m (8-ft) mast arm and light standard was selected for use withthe road closure gate design. If the mast arm and light standard areomitted from the design, a taller pole would be necessary to increasethe mass moment of inertia and to achieve comparable impact per-formance. For example, analysis indicates that WYDOT’s next-tallest standard luminaire pole, which is 11.4 m (37 ft 6 in.) high and


CONCLUSIONS AND DISCUSSION OF RESULTS

An existing road closure gate design used by WYDOT was analyzedand redesigned in the present study. The modified road closure gatedesign was successfully crash tested in accordance with guidelinesset forth in NCHRP Report 350 (1) and the 1985 AASHTO Stan-dard Specifications for Structural Supports for Highway Signs,Luminaires, and Traffic Signals (2). The road closure gate designhas been adopted by WYDOT and accepted by FHWA for use onthe National Highway System (Letter from L. G. Swanson,Wyoming Division, FHWA, April 12, 1995, to D. G. Diller, Direc-tor of WYDOT).

FIGURE 13 Damage sustained by test installation and vehicle in high-speed crash test.

FIGURE 12 (continued)

0.516 s

0.602 s

Mak et al. 51

which has an outside base diameter of 238 mm (9.375 in.), wouldfunction in a manner similar to the way the tested system did.Although other support pole options have not been tested, analysisindicates that a taller support pole structure would be an acceptablealternative.

• The height of the gate arm bracket was set at 5.5 m (18 ft)above the base of the support pole structure for the test installation.The mounting height of the gate arm bracket should be adjusted toaccommodate the actual length of the gate arm in use. However, aminimum mounting height of 5.5 m (18 ft) is recommended. Forlocations where high wind speeds pose a problem to the properretraction of the gate arm into the bracket, the length or the angle ofthe bracket, or both, can be increased to better accommodate theretraction of the gate arm. These increases should not adverselyaffect the impact performance of the road closure gate.

• The road closure gate design, as tested, uses a four-bolt slipbase for the breakaway mechanism. However, there is no reason tobelieve that the road closure gate design would not perform satis-factorily when used with other crash-tested and approved break-away bases, such as a three-bolt slip base, a frangible transformerbase, or frangible couplings.

• The road closure gate design, as tested, uses an electric in-linelinear actuator as the gate arm lift mechanism. Alternative lift mech-anisms, such as a manual winch and pulley mechanism, should notadversely affect the impact performance and may be used, providedthat they do not significantly add to the weight or the size of thedesign.

REFERENCES

1. Ross, H. E., Jr., D. L. Sicking, R. A. Zimmer, and J. D. Michie. NCHRPReport 350: Recommended Procedures for the Safety Performance Eval-uation of Highway Features. TRB, National Research Council, Wash-ington, D.C., 1993.

2. Standard Specifications for Structural Supports for Highway Signs,Luminaires, and Traffic Signals. AASHTO, Washington, D.C., 1985.

3. Pfeifer, B. G., J. C. Holloway, R. K. Faller, E. R. Post, and D. L. Christensen. Full-Scale Crash Test on a Luminaire Support 4-Bolt Slipbase Design. In Transportation Research Record 1367, TRB,National Research Council, Washington, D.C., 1992, pp. 13–22.

4. Swanson, L. G., Wyoming Division, FHWA. Letter to D. G. Diller,Wyoming Department of Transportation, April 12, 1995.

wyoming road closure gate - transportation research...

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