A New Concept of Traffic Measurement and Control

Before any meaningful measurement or control of traffic can be made it is essential to have an accurate and reliable means of measuring it. “To measure is to know ”, Lord Kelvin, 1824-1907. Current methods depend very heavily on manual counts using paper “tick sheets” and “clickers”, handled by retired pensioners. The results are often averaged and carried out at intervals varying from hours to days, weeks and sometimes years. It is unusual for such a census of traffic to be carried out during the hours of darkness, at weekends or Bank holidays. More sophisticated devices such as pressure hoses stretched across the carriageway, induction loops buried in the road, and various types of interruptible beams are also employed. By their own admission, the Highways Agency only place a 95% reliability on some of these items, naming installation, maintenance or sabotage as some of the problems. The form of the signal from the induction loops raises a number of queries for which it has been difficult, if not impossible, to obtain any answers. The varying amounts of ferrous material connected with vehicles must give very different signals between a car with an aluminium engine and a multi-axle lorry with a cast iron engine, carrying a load of steel girders. The steel reinforcement of the road has also been known to cause problems. Glass fibre optics in the form of strain gauges have also been tried as a means of detecting vehicles by looking at the waveforms produced due to variations in weight as vehicles pass over them; they too have variations similar those in induction loops. Measurements from induction loops, strain gauges and pneumatic tubes are inferred measurements; rather like measuring the differential pressure across an orifice plate and calling it flow. Direct measurements are far superior to inferred ones. The frequent use of words and phrases such as: empirical formula, weighted average times, historical times, probabilities, empirical observations, etc. all contribute to the confusion. Few positive measurements are mentioned. It is essential to be able to measure before attempting control, “Divide et Impera”our Institute motto.