Creating Tomorrow Today - Immovable Object: Preventing Bridge Strikes

Written by  Bill Hale P.E. TXDOT Chief Engineer

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Whether caused by permitted trucks that are improperly loaded, non-permitted vehicles with excessively large equipment, or dump trucks that are driven with their beds in the elevated position – which happens more often than you would think – over-height bridge strikes are a reality across the state of Texas. These strikes can endanger lives, do considerable damage, tie up traffic and require costly repairs. And at the Texas Department of Transportation, we’re working on ways to prevent such strikes before they occur.

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Chief Engineer Bill Hale gives an overview of TxDOT's forecast and how they plan to level out lettings to target $500 million a month.

Over-height bridge strikes create risk to the traveling public. Large pieces of debris can fall onto open lanes of traffic. In severe cases, girders, deck bays or entire bridge spans can collapse onto traffic. Our bridges are resilient and catastrophic collapses are highly infrequent but they do happen. Not surprisingly, the frequency of bridge strikes increases with upticks in the energy sector of the state since there is a great deal of large equipment being transported.

These over-height strikes cost a lot, both in terms of the price of repairs and traffic restrictions that occur after these strikes. Many of these strikes result in complete replacement of one or more girders and occasionally an entire bridge span must be removed and replaced. In such cases, extended traffic closures are inevitable. While TxDOT pursues reimbursement from the trucking companies involved in these incidents and their insurance carriers, much of the repair cost goes unrecovered. And particularly in West Texas, where there are often no convenient detours, extensive repairs can cause major traffic headaches for weeks or months. As a result, TxDOT is developing techniques that allow for rapid turnaround of emergency repairs.

TxDOT’s mission is to provide a safe transportation system, so one obvious solution to preventing over-height strikes is to raise the height of the bridges. As part of its ongoing effort to improve the Texas Highway Freight Network to enhance freight safety and mobility, TxDOT adopted a new vertical clearance standard last year that will be implemented on all new construction and reconstruction projects on the freight network – where practical and feasible – starting Sept. 1, 2020. The new standard is 18 feet, 6 inches, increased from the previous standard of 16 feet, 6 inches. If the new vertical clearance cannot be achieved, an alternative route will be identified. And where many older structures have vertical clearances significantly less than even the previous standard, it will take time for the new standard to be implemented.

Raising vertical clearances may prevent some of these strikes but it won’t solve the issue. Close to half the major strikes occur with clearance higher than 18 feet. As a result, TxDOT is exploring options for over-height detection and warning systems. Primarily these systems will detect an over-height vehicle and provide some warning to the driver. The warning needs to be intense enough to get a driver’s attention without alarming other motorists and far enough in advance of the bridge that a driver has the ability to exit, but not so far in advance that the warning goes unheeded.

Some TxDOT districts have already installed over-height detection sensors and warning signs with beacons which alert over-height drivers to exit before striking the bridge. Optional enhancements to that system include event data recording and the image capturing of trucks that violate the height restriction.

The Houston District has two systems that are equipped with the optional enhancements. The Fort Worth District is upgrading its system, and the Austin District is constructing three systems that will include certain optional enhancements.

With a state the size of Texas, tackling this issue is a challenge, but TxDOT is dedicated to developing and installing systems that reduce the likelihood of overheight bridge strikes statewide to better protect the traveling public.

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