Functionally Hierarchical Road Classification Considering the Area Characteristics for the Performance-oriented Road Planning

Abstract

Functional hierarchy of road network classifies individual roads into several levels by taking account of the priority for mobility, access or residential functions, in order to manage traffic efficiently by segregating through traffic from accessing, parking and non-motorized traffics. This concept has been widely recognized and applied to road planning, design and operation in several countries such as Germany, the US and Australia. Their guidelines state the target performances (i.e., LOS or travel speed in general) of each hierarchical levels being used for the performance evaluation. However in Japan, target performances of roads are not shown in the current guideline, since a concept of the performance-oriented planning and design is still uncommon in practice. As a result, the operational performances are still not satisfactory enough especially on highways. In such a situation, this study attempts to find out the hierarchical road classification applicable to Japan, considering the distribution of region/district centers as well as the impact of terrain. The main questions are how many hierarchical levels need to be defined and how much travel speed and road spacing (or access distance) should be set as the targets for each level, so that the network performance, namely travel time between region/district centers, can achieve the given target values. The basic idea comes from the German guideline RIN, which states the target travel times by region/district center (central place), classifies roads and define their target travel speeds based on their connecting centers. However, it does not clearly mention how the difference in distribution of region/district centers and the impact of terrain can be reflected to set the efficient hierarchical classification. For example, the characteristic of metropolitan areas is quite different from that of mountainous areas in terms of trip lengths between region/district centers of which travel times are to be evaluated as network performance. In addition, because of the impact of terrain, construction cost per unit is generally quite higher in mountainous areas than in flat areas, whereas travel demand is usually lower in mountainous areas. This study incorporates these issues into road classification and demonstrates its impact for more efficient network system planning. The methodology consists of four steps: (1) defining the region/district centers to be connected and setting their target travel time, (2) developing alternative road classification scenarios, (3) mathematically formulating origin-destination travel time in a grid road network under the certain classification and (4) evaluating the classification scenarios considering the difference of average trip lengths of each combination of centers as well as the difference of system cost due to the impact of terrain and travel demand. Although the original intention of this study is to apply the result to Japan, the methodology itself is generally applicable to any region. At the end, case studies are conducted for several simplified areas for reflecting their typical characteristics of region/district centers distribution and terrain in Japan. Through them, the hierarchical road classifications by area type are suggested as a conclusion.

Keywords

road network planning ; target performance ; functional hierarchy

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