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30 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 4.2 Dynamic Warning Signs 4.2.1 Overview Two NCHRP Project 3-74 test sites, one in Washington and one in Texas, provided docu- mented applications for the high-speed intersection treatments discussed in this section. Dynamic warning signs can alert drivers to the need to reduce their speed and encourage earlier deceleration. These treatments can be applied in rural, non-signalized intersections in advance of abnormal roadway features. Design variations include speed activation, message, image, size, and location. Driver workload and power supply issues should be considered. 4.2.2 Applicability and Considerations Dynamic warning signs are placed along the side of the roadway prior to a location that requires reduced speed. The signs are activated by vehicles that exceed a predetermined speed (typically in excess of the posted speed limit) or by potential vehicle conflicts at the intersection. This type of sign is not intended to enforce the speed limit; rather it is assumed that drivers will reduce their speeds once brought to their attention. (Maze et al., 2000) Although most com- monly applied in work zones, these signs have potential to be applied at intersections. Dynamic warning signs have been used at curve approaches, in work zones, and in other locations that require reduced speeds. Dynamic warning signs have also been used to warn drivers of conflicting cross-traffic at inter- sections. Called collision countermeasure systems (CCS) in this application, they are used to reduce side-impact crashes at rural non-signalized intersections. On low-volume rural highways with a history of high intersection crash rates, dynamic warning signs of this type are a cost- effective alternative to a conventional traffic signal. (Hanscom, 2001) Dynamic warning sign systems typically combine a radar device or pavement loop detectors with a variable message sign. The system measures the speed of the approaching vehicle or detects a potential vehicle conflict. The system provides a message to drivers who are traveling at exces- sive speeds or whose movements are in potential conflict with another vehicle. Examples of the types of messages displayed on the variable message sign include Slow Down, XX mph Curve Ahead, Your Speed XX mph or Traffic Ahead. (Torbic et al., 2004; Hanscom, 2001) Some systems are designed to provide messages to only a certain type of vehicle, such as trucks. In these cases, the dynamic warning systems involve weigh-in-motion devices, loop detectors, and height detec- tors. (Torbic et al., 2004) One of the challenges for implementing dynamic warning signs is to determine the maximum safe speed above which the sign will be activated. Vehicle loads, suspension, vehicle size, tires, and variable weather conditions can affect this speed. (Torbic et al., 2004) The maximum safe speeds selected by both Washington and Texas for the NCHRP Project 3-74 research test sites were higher than the posted speed limit. Dynamic warning signs can vary in complexity; therefore, some systems can be installed in a very short period of time, while others may take years to implement. The radar and video equipment, loop detectors, and weigh-in-motion detectors can be expected to encounter the same maintenance issues as when placed at conventional signalized intersections or other locations. When dynamic warning sign equipment and posts are placed outside of pedestrian and bicycle zones, they are not expected to impact multimodal users. Exhibits 4-1, 4-2, and 4-3 show three different types of dynamic warning signs.
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Treatment Descriptions 31 (Credit: WSDOT) Exhibit 4-1. Dynamic warning sign. 4.2.3 Treatment Layout/Design Dynamic warning signs are placed in a location that provides adequate advance warning time for drivers to reduce their speed appropriately. The placement of a dynamic warning sign should be determined from estimated perceptionreaction time, deceleration, and the stopping sight distance in advance of the intersection. Sign placement will reflect the desired target speed loca- tion and the driver deceleration rates from the operating speed of the upstream segment. Vertical and horizontal alignments must provide a clear sight line for the radar or video equip- ment. A power source is also necessary. (Torbic et al., 2004) Constraints such as steep slopes, bridges, and power supplies also should be considered to determine the appropriate location for the sign. If a radar unit is included as part of the dynamic sign treatment, it must be placed to detect drivers' speeds in advance of the sign. 4.2.4 Speed Effects At each NCHRP Project 3-74 test site in Washington and Texas, dynamic warning signs were tested at two approaches. Dynamic warning signs reduced speeds significantly at three of these four high-speed intersection approaches. Speed data were collected at three locations on each intersection approach. A mean speed reduction of 1.7 mph was observed at the sign locations
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32 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Photo: TXDOT) Exhibit 4-2. Dynamic curve-warning sign. (Credit: PennDOT) Exhibit 4-3. Dynamic warning sign used as a collision avoidance system.