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cost stabilizes at $25.5 million annually, i.e., $4,400 per kilometer<sup>of paved trunk road.3</sup> Given Ghana's weather conditions, heavy traffic loading,<sup>and poor maintenance record,</sup> about 70 percent of these costs ($3,100) can be attributed<sup>to traffic. Average daily traffic on</sup> paved roads was 788 vehicles. Assuming that the average loading<sup>is 1.2 ESAL per vehicle</sup> (an average of the high and low estimates), the annual pavement<sup>loading is on average about</sup> 350,000 ESALS. Thus, our second estimate of road user costs is about<sup>US¢0.9 per ESAL-km</sup> driven on paved roads.

The two estimates presented above share the<sup>common assumption that the timing of</sup> pavement resurfacing/strengthening is responsive to the state of<sup>the road. Given this</sup> assumption, additional traffic imposes only a minor externality<sup>on the rest of traffic, since</sup> the increased operating costs due to road damage are offset to a large<sup>extent by the reduced</sup> costs once the road has been overhauled. Unfortunately, the African<sup>experience indicates that</sup> this basic assumption is not always valid, and the timing of<sup>the road improvement is</sup> determined by budgetary, financial or political considerations<sup>which are only loosely related</sup> to the state of the road.

In this case, one cannot assume that additional<sup>traffic will advance the time of even</sup> overlay. An increase in traffic may involve an increase<sup>in the cost of strengthening or any</sup> required rehabilitation, and will definitely result in<sup>a serious road-damage externality in the</sup> form of increased vehicle operating costs.

The third estimate attempts to measure this<sup>externality directly. For this purpose one</sup> must estimate the effect of an additional veihicle<sup>on road roughness, and the effect of the</sup> increase in roughness on vehicle operation costs. Both<sup>estimates involve large inaccuracies.</sup> Local consultants (SWKP, 1988b) estimated that in 1988<sup>a one-unit increase in roughness</sup> (International Roughness Index, IRI) increases average<sup>vehicle operating costs by C1.4 per</sup> km.<sup>4</sup> The number of vehicles affected by the deterioration<sup>of the road depends on the state</sup> of the road, since the state of the road and the volume of<sup>traffic are negatively correlated.</sup> The average vehicle uses a road that is traveled by<sup>another 1,930 vehicles a day.' If it is a</sup> 3 The program (incorporated in the 1987 Transport Rehabilitation Program) envisages the annual resealing of 285 km of fair roads (at a cost of $22,000 per km), the resurfacing<sup>of 83 km of poor roads (at a cost of</sup> $50,000 per km), and a major rehabilitation of 60 km (at a cost<sup>of $250,000 per km). This program will</sup> preserve the steady state distribution of 70:30: 10 of good, fair,<sup>and poor roads.</sup> ' This estimate is considerably higher than Paterson's estimates<sup>for Tunisia (1985) and slightly higher than</sup> Chesher and Harrison's estimates (1987) for Brazil and India. It<sup>is lower than the estimates reported by Chesher</sup> and Harrison for Kenya and is in line with the GHA 1987 estimates<sup>(Bhandari and Abbey-Sam, 1988), which</sup> show a marginal effect of ¢1.23 per mi.

S Note that this average differs from the annual average<sup>daily traffic. The average daily volume per km</sup> **ADTK** per km is obtained by weighting the first column in Table 5 (panel<sup>B) by the percentage in column (2).</sup> l'he average daily volume per vehicle (ADTv) is obtained by weighting<sup>column (1) by the percentages in column</sup> (3). To demonstrate this difference, let a sys.em consist of 1,000 km,<sup>999 of which are traveled by 1 car and</sup> 1 km traveled by 1,001 vehicles. The **ArK** is two vehicles per km.<sup>However, the average vehicle travels on</sup> a road which is traveled by 501.5 vehicles or the ADTV is approximately:<sup>0.5 x 1 + 0.5 x 1,001.</sup>