From time to time, the Gauteng Department of Public Transport, Roads and Works is requested to permit additional interchanges on its freeway system. Gauteng, one of South Africa's nine provinces, is also the economic powerhouse of the country. As such there are tremendous development pressures and Gautrans felt obliged to re-evaluate its approach to interchange spacing. This re-evaluation comprised a literature survey, limited field observations on existing freeways, a review of signposting requirements, weaving analysis, and other operational considerations, and formed the basis of the paper presented at the 2nd International Symposium on Highway Geometric Design, in Mainz, Germany in 2000. The revised approach is based on the distances necessary to accommodate the sign sequences recommended in the Road Traffic Signs Manual of the Southern African Development Community, but recognizes that the distances derived on this basis may be too short to accommodate the anticipated flows at the proposed interchanges, in which case the distance required for operational efficiency will have to be determined. It may also be necessary for the benefit of the surrounding land use to consider the provision of an interchange at a less than normally desirable spacing, again requiring an investigation of operational efficiency. The approach to operational analysis followed in the 2000 paper was based in part on the then current Highway Capacity Manual (HCM 1994), but also included reference to research work being undertaken at the time for inclusion in the HCM 2000. However, when released, HCM 2000 did not include this research work. HCM 2000 is also significantly different from its predecessor and it was accordingly deemed advisable to re-evaluate this aspect of the original proposals. Exhaustive analysis utilizing the methodologies expounded in HCM 2000 for operational analysis, led to the development of a series of graphs for the merge-diverge case, from which the distance (spacing) required to maintain acceptable operational conditions can be determined for a large range of operational circumstances. A density of 22 veh/km/ln or, with a relaxation, 25 veh/km/ln, is proposed as lower limit. If a merge-diverge is inappropriate, a weaving section will have to be evaluated. A "warrant" approach using a Type A or a Type B configuration is proposed as the basis of evaluation, and graphs have been developed from which the densities for given flows can be read. These graphs are based on the assumption of volume ratios (VR) of 0.27 and 0.60 for Type A and Type B weaving, respectively. These values of VR are three-quarters of those at which, according to HCM 2000, instability of the weaving section is likely to develop. It has also been found that for both Type A and Type B weaving, the presence of the auxiliary lane renders the question of a minimum distance between interchanges largely irrelevant. The point at issue is whether the anticipated density in the proposed weaving sections will be less than the recommended levels.
Abstract