Lecture 40 Traffic Flow and Volume Characteristics Flow Theory Volume is the number of vehicles that pass a point in a unit of time. The basic relationship for volume is sukuq u is the space mean speed and k is the concentration (veh/mi). Note that the spacing (mi/veh)is the inverse of the concentration and theunits for volume are in vehicles/hour.The relationship between concentration and speed is not known and many relationships have been proposed. The validity of any relationship must be proved by actual measurements of traffic flow conditions. One relationship proposed by Greenshield is that speed is linearly related to the concentration. For example:5.350ku This is the equation of a straight line with an intercept of 50 and a slope of 1/3.5. By rearranging this takes the formuk 5.3175 also by substitution )5.350()5.3175(kkqanduuq These equations produce curves of the following shapes.u50uq218725Basically it is seen that as speed increases the concentration decreases until it reaches a maximum (jam concentration) at a speed of 0 mph. Volume increases with speed up to a point and then decreases to near zero at so called free flow.k 175 q
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