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The difficulty with the procedures such as those published by Anderson and others (1975) and Tseng (1975) is that the unit costs used in developing the regression equations and monographs increase rapidly. In many cases, the risk component due to traffic will be small. Where severe grades, long detours, or extreme speed differences between the main road and detour exist, a detailed evaluation of running costs may be in order. A reasonable approximation of the cost of the detour can be determined by using the measured traffic volume multiplied by a vehicle cost per mile and the added mileage. FHWA annually publishes figures on the vehicle cost per mile for the various classes of vehicles. From these figures, the cost per mile can be determined for the various traffic mixes.. Traffic normally increases from an initial low volume to a projected high volume. For purposes of the economic analysis, it is reasonable to assume that the traffic volume will be a gradually varying series. A gradually varying series can be converted to an equivalent uniform annual series by procedures described by Grant and Ireson (1960). The equivalent uniform annual series for the average daily traffic, ADTE, that represents a growing traffic series is: ADTE = ADTl + G(gf) (2) where ADTl = initial ADT at the end of the first year G = growth rate of the traffic volume (ADTN - ADTll/n ADTN = projected ADT at the end of n years gf = factor to convert a gradually varying series to an equivalent annual series 1 gf = (1 i = interest i n + i)n rate - 1 Since traffic may be interrupted due to flow over the road, user cost decreases as embankment height increases as shown in figure 5. In general, as bridge length increases, more water will go through the bridge and less over the road. Therefore, cost of traffic interruption will decrease as bridge length increases. For a given discharge, when all the flow goes through the bridge, no traffic interruption will occur. Embankment Repairs.--When road fills are overtopped, some erosion occurs. The amount of erosion is not predictable by available methods. However, there is the question of how much it will cost to replace sections of the embankment if it does wash out. The availability of fill material, haul distance, the cost of equipment, and even the cost of placing temporary structures should be considered. In these studies embankment erosion was assumed to vary with duration and depth of embankment overflow. 22