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Proper traffic control & delineation is critical to achieving safety in work zones. However, the work zone traffic control devices themselves may pose a safety hazard to vehicle occupants or work crews when impacted by errant vehicles. Thus, the Federal Highway Administration (FHWA) and the Manual on Uniform Traffic Control Devices (MUTCD) require that the crashworthiness of work zone traffic control devices be demonstrated before they are implemented on the nation’s highways.
Guidance for evaluating the safety performance of work zone traffic control devices is contained in National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” NCHRP Report 350 represents a comprehensive update of the procedures for crash testing of both permanent and temporary highway safety features and the evaluation criteria used to assess the test results. These guidelines reflect an evolution of knowledge in this area over the last 30 years and incorporate current technology and the collective judgement and expertise of roadside safety professionals.
Before the publication of NCHRP Report 350, test matrices for work zone devices were not well defined. As a result, little crash testing was conducted and the impact performance of many commonly used devices was largely unknown. Thus, there was a need to research the safety performance of work zone traffic control devices to assure they perform satisfactorily and meet the new NCHRP Report 350 guidelines which have been formally adopted by FHWA by a final rule in the Federal Register.
The test matrix for work zone traffic control devices consists of two tests with an 820-kg passenger car: a low-speed test and a high-speed test. For test level 3, which is defined as the basic test level, the relevant test designations are 3-70 and 3-71, which have design impact speeds of 35 km/h and 100 km/h, respectively. However, NCHRP Report 350 allows the omission of the low-speed test (test designation 3-70) when it can be clearly determined that the high-speed test (test designation 3-71) is more critical. This is often the case for various work zone traffic control devises having a relatively small mass due to the increased propensity for occupant compartment intrusion at higher speeds.
The evaluation criteria contained in NCHRP Report 350 to assess the performance of the work zone traffic control devices consist of several factors:
- Occupant risk measured in terms of occupant impact velocity (limit – 5 m/s) and ridedown acceleration (limit – 20 g’s). For devices with relatively small mass, these criteria are often well below the recommended limits and, therefore, instrumentation of the test vehicles for purposes of computation of occupant risk is not always necessary.
- Occupant compartment integrity. Of primary concern regarding the impact behavior of a work zone device is penetration of the test article or parts of the test article into the occupant compartment. In order to minimize the potential for injury during impact, there should not be any penetration or intrusion into the occupant compartment. Further, the windshield should not be shattered or damaged to the extent that it obstructs the vision of the driver.
- Test article debris. Debris from the test article should not pose any potential hazard to the vehicle occupants, other traffic, pedestrians, or workers in the immediate vicinity.
- Vehicle Stability. The test vehicle should remain upright and stable throughout impact sequence, i.e., both during and after the impact.
Written by Roger Bligh, Manager, Highway Safety Structures Program, Texas Transportation Institute, September 1998.
Last modified: 9/21/98