Locations which would benefit from the installation of supplemental advance warning devices typically exhibit safety and/or operational problems. Establishing the need for supplemental devices, therefore, requires identifying the problem locations and performing a safety and/or operational analysis. Deficient locations can be identified by a traffic safety management system, citizen complaints, employee observations, and by safety analysis during a planned resurfacing, restoration, and rehabilitation (RRR) project.
Accident-based studies are used to identify locations that can be considered hazardous due to a large number of accidents. These studies involve the review and analysis of systemwide accident information. To compare the accident experience of several locations, the length of time over which accidents are counted, the traffic volumes, and the length of roadway section involved should be the same at each location. If not, accident rates may be compared between locations, provided that a common unit of exposure (e. g., accidents per million vehicle miles for longer roadway sections, or accidents per million entering vehicles for spot locations and intersections) is used.
Potential locations can also be identified by complaints received from citizens and by observations made by employees. Often a combination of accident analysis and an investigation of complaints and observations is required for low-volume roadways. Complaints about “near misses” and observations of hazardous roadway elements can be considered indicators of site deficiencies. This type of information is treated by some agencies with the same importance as a documented accident history. Such treatment has the advantage of reducing the number of accidents required to identify the hazardous roadway locations.
It should be recognized that maintaining a complaint and employee observation file requires that the agency be responsive to these inputs. Complaints and observations are notifications of hazards that become a matter of public record and are available as evidence should an accident result in litigation. This alone is not a valid reason to fail to maintain a complaint and observation file. If a defect is allowed to remain for an unreasonable period of time, even if no complaints or observations were received, the courts can consider it as constructive notice and assign liability. Complaint and observation files should, therefore, be maintained and a program established to respond to all complaints and to document facts and engineering decisions to minimize the possibility of lawsuit losses.
An opportune time to identify the need for a device is during the design phase of projects primarily intended to upgrade the physical and operational characteristics of the roadway. This opportunity can be used to detect safety and operational deficiencies and to select appropriate improvements that can be incorporated into the upgrading project.
The identification of potential locations for each of the previous methods should include a field inspection to help establish the cause of the deficiency and appropriate countermeasures. If the site inspection indicates that the deficiency cannot be readily corrected due to cost or physical constraints, then an advance warning device should be installed. If the site conditions are sufficiently unusual that an appropriate warning device is not contained in the federal or appropriate state MUTCD, then a supplemental device may need to be used or developed until it is feasible to take care of the underlying problem.
For example, consider a situation where a sag vertical curve was constructed to provide sufficient vertical bridge clearance on a roadway with a posted speed of 45 mi/h (70 km/h). Analysis of the areawide accidents indicated that there is a higher than expected occurrence of intersection-related and rear-end accidents at a signalized intersection immediately downstream of the bridge. A visit to the site indicated that the signal faces were not visible to approaching drivers until they were 400 ft (120 m) from the stop line. Since this distance is less than the minimum visibility distance of 460 ft (140 m) specified by Sec. 4D.15 of MUTCD, a Signal Ahead sign (W3-3) was installed [2]. The engineer determined that, although the minimum recommendations of MUTCD were being achieved, safety improvements could be achieved by providing real-time warning that a stop will be required at the intersection. Since removing the sight obstruction was not
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FIGURE 7.1 Example of supplemental advance warning sign. |
possible, the engineer considered lowering the speed limit and/or providing additional motorist warning. Experience with lowering speed limits indicated that this countermeasure was not an effective long-term solution. The engineer decided to install an active supplemental advance warning device with the legend “Prepare to Stop When Flashing” configured as shown in Fig. 7.1. The device was installed over the roadway, 500 ft (150 m) in advance of the stop bar, and interconnected with the traffic signal controller. The horizontally mounted beacons were timed to flash yellow 8 s prior to the red indication so that drivers passing the beacon at the legal speed limit would have advance warning of the required stop at the intersection. The yellow beacons continued to flash until 3 s before the end of the red indication to allow the start of queue dissipation. Motorists not encountering the flashing lights could expect not needing to come to a complete stop at the signal, while still having the signal presence reinforced by the overhead sign. The engineer plans to continue monitoring the location to determine if the active advance warning device is effective in reducing accidents.
