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Accident Reconstruction Research: Guardrails

Effects of Presence of Light Poles on Vehicle Impact of Roadside Barriers
by James Kennedy (Battelle Memorial Institute, 505 W. King Ave., Columbas,OH 43201-2693)(Aug 1995).
[TD 100: OH 95-17]
This report evalutes whether the presence of light poles immediately behind guardrails reduces safety. The conclusion is that it does not.

When light poles are installed within the deflection zone of guardrails, they may influence the ability of the guardrail to redirect an impacting vehicle safely. Although most highway light poles are designed to "break away upon impact," insufficient data exist to verify that as a vehicle deflects a guardrail into a light pole, the light pole breaks away effectively.

A concern is that light poles may act in a manner similar to bridge abutments. There is evidence that in crash situations where the vehicle pushes the guardrail into the abutment, it generally pivots about the relatively rigid abutment and is injected back into the traffic stream in an uncontrolled, unsafe manner (for example, spinning). While the size, mass, and stiffness properties of a light pole are much different than a bridge abutment, they may be sufficient to degrade the redirecting performance of the guardrail.

If light poles have an adverse effect on the performance of guardrails, then significant and costly modifications and replacements may need to be made to existing barriers and light poles. Required actions could include moving existing light poles out of the deflection zone of the guardrail, stiffening the guardrails to reduce their maximum deflections (which in turn could degrade their redirecting performance), retrofitting existing light poles with new, more frangible bases, and replacing entire light pole assemblies.

To shed light on these concerns, particularly safety and potential costs, the Ohio Department of Transportation (ODOT) in conjunction with the Federal Highway Administration (FHWA) authorized a project that included full-scale testing and limited modeling to assess if and how existing light pole designs affect the redirecting performance of guardrails under vehicle impact conditions.

Four pickup truck-style (2000P) vehicles were impacted at an angle of 25 degrees and a speed of 100 km/h (60 mph) into a guardrail/light pole system typically employed in Ohio. All tests conformed to the requirement of NCHRP Report 350. The location of the impact position relative to the position of the light pole along the guardrail length of need varied according to the specific requirement of each test. None of the impact scenarios resulted in snagging of the 2000P vehicle with the guardrail/light pole. Additionally, the 2000P vehicles showed no tendency to pitch, roll, ramp, or vault throughout contact and exit with the guardrail as a result of the impact. However, excessive exit angles were observed in three of the four 2000P vehicle field tests.

Two small cars (820C) were also impacted into the guardrail/light pole system at an angle of 20 degrees and a speed of 100 km/h, which is consistent with the requirements of NCHRP Report 350. Neither of these tests indicated a tendency for snagging of the 820C vehicle with the guardrail/light pole system as a result of impact. It can therefore be concluded that for the vehicle type tested, the guard/light pole system typically used in Ohio does not cause snagging and/or subsequent unstable motion of the vehicle due to impact. The vehicles (both 820C and 2000P) exited the guardrail in a stable manner after impact.

From the results of the project, no changes in the arrangement of light poles behind the Type 5 guardrail is recommended. The redirecting function of the guardrail was not compromised as a result of placement of the light pole behind the guardrail. The field tests and computer model results indicated that the redirecting function was for placement of the light pole at various distances from the impact point. The field tests also indicated that the functional integrity of the guardrail was maintained for both base designs ("X" and "A") prominent in field usage in Ohio.

An evaluation of excessive exit angle situations that occurred in three out of the four field tests involving the 2000P vehicle may be warranted. However the guidelines in NCHRP Report 350 regarding exit angle values are based on impact of the vehicle with the barrier and not the barrier and the light pole. Therefore this situation must be considered when judging the behavior of the system regarding exit angle values. Also the exit angle values from these three tests should be viewed considering the fact that the impact conditions employed for these tests were extreme, and the likelihood of this situation occurring under everyday highway usage may be small.