The primary objective of the research described in this report was to provide new and improved deterioration relationships for unpaved roads for use in a computer model for estimating the construction costs, maintenance costs and vehicle operating costs of roads in developing countries. Experimental test sections were located on the public road network in Kenya and their rates of deterioration were measured and quantified in terms of gravel loss of the wearing course, surface roughness, depth of loose surface material and rut depths. The test sections were monitored for a period of over 2 years. The deterioration of the test sections was related to cumulative traffic, original design standards and construction, maintenance strategy and climate.