- 24 -

The first of these functions applies to a 2-lane, 2-way suburban road, the second to a 2-lane, 2-way urban street, and the third to a 2-lane, 2-way **CBD** (Central Business District) street (the regressions are presented in Figure 3).

To estimate the marginal tinr costs one has to know the average traffic volume (or, alternatively, the average speed) and the u stribution of traffic conditions (i.e., the fraction of traffic subject to the various speed-flow conditions). Traffic counts indicate that the average traffic volume on Accra streets is 800 vehicles per hour, the average for Tema is 500 and for Takorad;: 300-400 (we could not obtain the data for Kumasi, the second largest urban center was not available). It was assumed that the first function is typical of traffic conditions in the outskirts of most big towns (about 10 percent of urban traffic), the second of traffic conditions in the CBDS of most towns and the inner circle of Accra (80 percent of traffic), and the third to Accra's CBD (10 percent of traffic), and that the traffic volume is _X_ = 500, 500, and 800, respectively. Applying these traffic volumes to the three speed-flow equations one obtains estimates of the average speed of 66.2, 51.7 and 39.0 km/hour. The marginal time costs are accordingly 0.0013 hours per vehicle for the low-congestion conditions, 0.0041 for the medium case, and 0.0044 for the center of Accra. By this estimate the average MTC is 0.00385 hours per vehicle (i.e., each additional vehicle imposes on the rest of the traffic a delay of 0.23 minutes). This value is 1/8 of the value assumed by Newbery in the Tunisian case.

Given the composition of urban traffic, average crew costs are 40¢ per hour. An evaluation of the value of lost passenger time requires knowledge of the distribution of traffic and the load factors of the various types of vehicles. Table 10 contains data on the occupancy rate of the different vehicles, based on an origin-destination survey conducted by DeLeuw-Cather in Accra. According to these data the average occupancy rate is 5.6 passengers per vehicle. Assigning a value of one third of hourly earnings to the value of time implies a value of 47¢ per vehicle-hour (= 25¢ X 0.33 X 5.6).

Using an estimate of MTC of 0.00385 hour per vehicle, the implied marginal congestion costs are 0.15¢ per vehicle-kni, if one allows only for crew time, and 0.34¢ if one also allows for the value of passenger time.

Finally, the effect of the different vehicles on urban congestion is not the same. A light truck counts as 1.5 cars, a medium truck as 2.0, and heavy trucks and basis as 2.5-3.0. Given the distribution of traffic on urban roads (and specifically, the large share of light vehicles in urban traffic), the average vehicle counts as 1.16 private cars (PCE). The marginal congestion costs are, therefore, 0. 13-0.29c per PCE-km (depending on whether one allows for the value of passengers' time or not). By comparison, Newbery's estimate was almost 15 times higher (i.e., 4.6¢ per PCE-km), reflecting both the greater congestion on Tunisian urban roads and the higher value of time. Newbery's estimate is 23 times the estimate of the interurban MCC. In the Ghanaian case, given the lower congestion on urban streets, the ratio is 10:1.