- 23 -

state of the road. SWKP (1987) note that when<sup>the road deteriorates to a level of roughness</sup> of 10-13 **iRi,** speed slows down to 30-50 kph.

The slowdown _per se_ tends to increase marginal<sup>congestion costs, but it is not clear how</sup> the marginal effect of traffic on speed ((3)<sup>is affected by the state of the road. On the one</sup> hand, overtaking is much more complicated<sup>on a poor road, which will tend to increase</sup><sup>_(,_</sup> but on the other hand, on seriously damaged<sup>roads all traffic (regardless of type of vehicle)</sup> tend to maintain the same slow pace and marginal<sup>changes in the volume have no effect on</sup> travel time. The sign of the interaction term<sup>between road roughness and traffic volume</sup> cannot, therefore, be ascertained: it can be<sup>either positive or negative. Assuming the estimate</sup> of 6 is unchanged by the state of the *oad,<sup>and that it affects only the free flow speed of</sup> vehicles, the estimates of MCC should be<sup>doubled when average speed in the system is 50</sup> km/lI (rather than 70 km/h) and quadrupled<sup>when average speed falls to 35 km/h. In either</sup> case, interurban congestion costs are low<sup>compared with the road damage estimates (and low</sup> compared with Newbery's estimate).

Different types of vehicles contribute differently to<sup>congestion, the effect increasing with</sup> the slowness and size of the vehicle. It is<sup>customary to assign values reflecting these</sup> differences (compared to private cars). The<sup>weight assigned to pickup trucks is 1.00 to 1.50,</sup> to minibuses--1.5, to heavy buses--2.0, to medium<sup>trucks--1.7 to 2.0, and to heavy trucks--</sup> 2.0 to 3.0. Given the distribution of traffic,<sup>the average passenger car equivalent (PCE) per</sup> vehicle on Ghanaian trunk roads is 1.4-1.9.<sup>Assuming a factor of 1.65, the long-run</sup> marginal congestion costs per PCE are CO.01-0.03.<sup>The short-run MCC may be 2-4 times</sup> higher than that.

Urban congestion is in general a much more<sup>serious problem compared with the con-</sup> gestion on interurban roads, as is the difficulty<sup>encountered in its measurement. Transport</sup> economists disagree on the relevant speed<sup>function, traffic counts on urban streets are much</sup> more scarce and so is the information on<sup>travel time. Thus Newbery, noting the high</sup> sensitivity of the results to the speed flow<sup>function, adopted arbitrary measures for the</sup> marginal time costs (MTC). Assuming this value<sup>is 0.1 per PCE-km in highly congested areas,</sup> 0.04 for moderately congested areas, and 0.02<sup>for lightly congested areas, and assigning</sup> weights of 0. 1, 0.1 and 0.8, respectively, to<sup>each of these congestion conditions, the average</sup> MTc estimated for Tunisia was 0.03 hours<sup>per PCEkm (i.e., each additional vehicle imposes</sup> an overall 2-minute delay on the traffic on<sup>the road traveled).</sup> This study was unusually fortunate in having<sup>at its disposal a study of speed-flow</sup> relationships on urban roads in Ghana. Following<sup>an extensive study of urban traffic by the</sup> GHA with the help of the University of Science<sup>and Technology (Kumasi), Bhandari (1990)</sup> estimated three speed functions for different<sup>congestion conditions on Accra roads:</sup>

- (a) S = 71.9 - _0.0115X_ (R<sup>2</sup> = 0.34) (b) S = 62.6 - 0.0219X (R<sup>2</sup> = 0.74)

(c) S = 45.7 - 0.0084X (R<sup>2</sup> = 0.46).