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Weighing-in-Motion Site Selection

To get the most beneficial degree of performance from a WIM system it is critical to ask yourself some questions about the purpose of the system and to select the site well. There are many factors to consider in site selection - among them are traffic patterns and road condition. To ensure the best site possible, ASTM has published a specification (E1318-94) which documents the pavement smoothness and other requirements for weigh-in-motion (WIM) systems. The road smoothness is critical for WIM systems, the more the vehicle is bouncing, the greater the error can be. The ASTM specification deals with the roadway 150' in advance and beyond the WIM scale, so the vehicle has time to stabilize as it is being weighed.

It is important for you to get to the proposed site before installation of the road sensors. In most cases, you can define a segment of where the site may be located to get the information you require rather than an exact milepost location. For instance, if on a freeway and there is limited access to the freeway, you may define that the site needs to be located anywhere between two exits. Try to find a section of pavement that is as smooth, straight, and level as possible. You may get lucky and find convenient power and phone drops close by, but please do not allow these items to dictate where the site is located. Understanding the construction and materials used for the road are critical, too. The system needs to be properly supported in the road and have a firm foundation. Pavement flexing can not only cause premature system failure, but also may cause inaccuracies in weight measurements.

Secondly, observe the traffic patterns at the proposed site; doing this can reduce inaccuracies caused by vehicles changing lanes often. Look for any factors which may effect the way traffic flows over the installed system. Be especially watchful for on/off ramps, rest areas, merging areas, or areas where motorists have a tendency to be changing lanes often - you want to try to ensure the traffic you need to monitor is hitting your sensor array as cleanly as possible. All these items can cause vehicles to 'straddle' the sensor array - causing inaccuracies in your data.

Furthermore, observe the surrounding area - things like overhead power lines or other transmission equipment or towers can produce interference that could effect some sensor technologies. Also, railroad tracks, bridges, or under road drainage tunnels can introduce errors as well. Of course, there are many other factors about the site itself which can effect the system accuracy.

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