Cover: Reliability Ratio’s voor het Goederenvervoer

Reliability Ratio’s voor het Goederenvervoer


Published Aug 26, 2005

by Marco Kouwenhoven, Gerard de Jong, Piet Rietveld

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Taking into account improvements of the reliability of travel times in cost-benefit analyses (CBA) of infrastructure investments is likely to become standard practice in the Netherlands. In order to make this possible, it is necessary to have a well-defined reliability measure that can be easily translated into monetary units. There are several methods of defining such a reliability measure. Rijkswaterstaat Adviesdienst Verkeer en Vervoer (the Transport Research Centre of the Dutch Ministry of Transport, Public Works and Water Management) prefers to use reliability ratios in valuation methods for reliability improvements because of international standards and because these ratios are needed as an input for LMT-BT (a tool to predict future reliability levels). Indicative values for the reliability ratios in passenger transport have been set before. The report at hand provides indicative values for the reliability ratios in freight transport which are based on earlier research on travel time valuation. Reliability ratios cannot be directly derived from this earlier study, since the buffer time and its relation with the time frame within which a delivery is called “in time” was not investigated. As a consequence, the current project makes assumptions on the ratio of the average transport time and the standard deviation of arrival times. For road transport, we can use an estimated ratio that is based on a different study, which is, however, only an approximation of the true value. Furthermore, an assumption was made on how freight carriers respond to an improved reliability: do they keep their buffer time constant, or will they postpone their departure to keep the probability to arrive “in time” constant? No research on this behaviour of freight carriers exists, so assumptions needed to be made. For road transport, we assume that 50% of the carriers keep their buffer time constant and 50% of the carriers keep their probability of arriving late constant, resulting in a reliability ratio for road transport of 1.2. This ratio depends strongly on the assumptions made, but can be used as an indicative value. The reliability ratio for other transport modes is more difficult to determine, since there is no information that can be used for estimating the ratio between transport time and the standard deviation of arrival time. However, it is possible to use a direct method to valuate reliability improvements, though this method does not use reliability ratios. In addition, the reliability ratio is determined for a wide range of assumptions on the ratio between transport time and the standard deviation of arrival time, and on the behaviour of freight carriers.

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