The roughness of a road's surface is an important measure of road condition and a key factor in determining vehicle operating costs on poor quality surfaces. This report describes a simple roughness measuring machine which has been designed especially for use in developing countries. It is called MERLIN - a Machine for Evaluating Roughness using Low-cost INstrumentation. The device can be used either for direct measurement or for calibrating response type instruments such as the vehicle-mounted bump integrator. It consists of a metal frame 1.8 metres long with a wheel at the front, a foot at the rear and a probe mid-way between them which rests on the road surface. The probe is attached to a moving arm, at the other end of which is a pointer which moves over a chart. The machine is wheeled along the road and at regular intervals the position of the pointer is recorded on the chart to build up a histogram. The width of this histogram can be used to give a good estimate of roughness in terms of the International Roughness Index. Calibration of the device was carried out using computer simulations of its operation on road profiles measured in the 1982 International Road Roughness Experiment. Merlins are in use in a number of developing countries. They can usually be made locally at a current cost of typically 250$ US.

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