DEVELOPMENT OF A WEIGH-IN-MOTION NETWORK IN SOUTH AFRICA

There is a system of comprehensive traffic observation (cto) in south africa that already provides a range of traffic flow statistics. The range includes rough estimates of road loads, in terms of thenumber of axles, axle masses, and the equivalent 80-kn axles (e80), obtained from samples. Recent developments such as the implementation of toll roads and the introduction of a campaign against overloaded trucks have created a need for more accurate data on road loads obtained by continuous weighing of trucks in motion. A truck-monitoring scale, which is a cto station with added sensors such as bendingplates, capacitive mats, or piezoelectric strips, was conceptualized. An experimental site with all the above-mentioned types of sensors was built, and their performance and accuracy were studied during a 5-month period. It was concluded that, given proper calibration, all sensors are sufficiently accurate for the purpose required, whichis to supply pavement engineers with measured road loads, to assistwith revenue audits in the vicinity of toll plazas, and to inform the law enforcement authorities of the degree of overloading on a given road. The precision of the scales was evaluated in terms of 95% confidence limits for the errors of individual axle mass measurements. The precision ranged from plus or minus 18% for the prozess-automatisierung technik plate (which is the most expensive sensor), to plus or minus 28% for the hard-top capacitive mat and plus or minus 38%for the piezoelectric strip (the least expensive one). Other statistics were comparable for these three scales. Although the evaluationclearly showed the importance of calibration for the accuracy of this type of truck scale, the calibration procedures have not yet beenperfected and require further work. It has been proposed that threesites be used for the improvement of the calibration and the assessment of the long-term performance of the different scales. This paper appears in transportation research record no. 1272, Pavement management and rehabilitation 1990.