An important area of concern relative to highway safety is the occurrence or drivers going the wrong way on one-way streets, highway mainline lanes, or highway entry or exit ramps. Along with significant signing, various monitoring approaches have been tried and implemented to provide immediate detection of vehicles going the "Wrong Way". To date, systems using magnetic induction loops represent the most common solution for "Wrong Way" detection (Figure 1). Two or more loops are placed in the roadway as shown in Figure 1. Relative to the correct direction of travel, loop #1 is the upstream vehicle detection sensor and loop # 2 is the downstream vehicle detection sensor. In the presence of traffic, vehicle detection at loop # 1 followed by detection at loop #2 indicates the correct direction of travel by the detected vehicle. Vehicle detection at loop #2 followed by detection at loop #1 indicates the incorrect direction of travel by the detected vehicle. When properly installed and maintained, the loop-based system performance should be reasonable.

Figure 1 Loop Based Wrong Way Detection

The SmarTek Acoustic Sensor (SAS-1) is absolutely suitable for "Wrong Way" detection of vehicle. The SAS-1 is designed to operate and provide effective and accurate vehicle presence detection for vehicles passing the sensor station at any reasonable (and allowable) speed from very slow (or stationary) to high speed. There is no inherent upper speed limitation for highway traffic because faster moving vehicles create increased levels of acoustic energy making them easier to detect. Because the SAS-1 is based on passive acoustic detection of motor vehicles, there is virtually no loss of detection performance due to variation in weather or environmental conditions or visibility conditions. For a typical "Wrong Way" detection system installation as shown in Figure 2, the maximum number of zones monitored by the SAS-1 is set at five(5). For special or custom installations, the SAS-1 can provide many more detection zones if necessary. Inherent in this configuration, is the capability for the SAS-1 to measure per vehicle speed. This capability is not included as a standard output at this time. For the configuration shown in Figure 2, a vehicle traveling in the correct direction will be detected in the SAS-1 detection zones in the following sequential order: zone 1, then 2, then 3, then 4, and finally zone 5. A vehicle traveling the "Wrong Way" will be detected in the SAS-1 detection zones in the opposite sequential order as follows: zone 5, then 4, then 3, then 2, and finally zone 1. Obviously, the spacing of the zones will determine the response time for the "Wrong Way" detection. The number of zones used is selectable, however, using more than two (2) is recommended to minimize the chance of false detections.

Figure 2 SAS-1 "Wrong Way" Detection Configuration

Using the easy to install side mount configuration, the SAS-1 provides a "Wrong Way" detection capability equivalent to a loop based system using up to five (5) loops.

Figure 4 SAS-1 Real Signal "Wrong Way"

Example - Using SAS-1 to Measure Per Vehicle Speed

Consider again the real acoustic signals shown in the display of figure 4. The previous discussion illustrated how the direction of the acoustic track (light green) and hence, the order that the vehicle presence relays become active indicates direction of travel (upper right to lower left or upper left to lower right). If one measures the actual value of the slope of the acoustic track (green track on the display) as it passes the SAS-1, then per vehicle speed can be accurately determined from the value of the slope. A fast moving vehicle has a large slope (nearer horizontal on the display) while a slow moving vehicle has a small slope (nearer vertical on the display). Note that just as in-pavement loops are spaced in the up/down road direction, each detection zone for SAS-1 corresponds to a physical location in the up/down road direction. Therefore, measuring the time between SAS-1 zone detections (relays become active) is the same as measuring the slope of the acoustic signal track and corresponds exactly to measuring time between loop detections in a double loop speed trap. One major difference, however, is that for the SAS-1 approach, we have four (4) time measures (between zone 1 and 2, between zone 2 and 3, etc.) to use for averaging instead of just one. This will result in a more accurate per vehicle speed measurement.