Empirical Studies on Zonal Capacity of Urban Activities Estimated by Maximum Capacity of Road Network

Asakura, Yasuo · 1988 · Crossref

DOI: 10.11361/journalcpij.23.385

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Summary

This paper addresses the need for a metric to evaluate the capacity of urban activities at the zonal level, rather than just the network level, based on the maximum capacity of the road network. The author argues that while the interdependence between traffic infrastructure and land development is well-recognized, existing measures of network capacity generally apply to the entire city. To properly assess the effects of road network improvements and determine appropriate levels of urban activity, a zonal indicator is required. The study aims to define "zonal activity permitted capacity" as the difference between the maximum traffic a zone can generate or attract (based on network capacity) and the currently generated trips, and to validate this measure using empirical data from Osaka City. The methodology employs an incremental assignment simulation to calculate the maximum network flow. The procedure involves iteratively loading Origin-Destination (OD) traffic onto the network until connectivity is lost. Link travel times are calculated using the Bureau of Public Roads (BPR) function, and links are considered "cut" (assigned infinite travel time) when their congestion ratio exceeds a specified threshold ($R_{max}$). The maximum capacity is defined as the total trip volume loaded just before the network becomes disconnected for specific OD pairs. Zonal capacity is then derived by aggregating these maximum flows at trip ends. The study applies this method to a large-scale network in Osaka, comprising 3,395 links and 1,120 nodes for the current state, and a projected future network with approximately 1.5 times the scale. Various scenarios were tested by adjusting $R_{max}$ (1.0, 1.5, 2.0) and the scope of OD pairs considered for connectivity checks. The results indicate that the current road network in Osaka is operating near its capacity limits. Under strict conditions ($R_{max}=1.0$), the network’s maximum capacity is significantly lower than current traffic volumes, suggesting that the current service level is already compromised. More realistic scenarios ($R_{max}=1.5$ or 2.0) yield maximum capacities closer to current volumes, confirming that the network is saturated. Comparing the current and future networks, the study finds that while the overall maximum capacity increases with planned infrastructure improvements, the increase in zonal activity permitted capacity varies significantly by location. Zones in the city center and coastal areas show substantial increases in permitted capacity, whereas zones in the eastern suburbs show little to no increase. This suggests that future road improvements will primarily benefit central and coastal development potential. The significance of this research lies in providing a planning-oriented metric for urban development that is independent of economic phenomena. By defining zonal capacity based on physical network constraints, the proposed indicator can serve as a reference for determining floor area ratios and guiding land-use policies. It allows planners to identify zones where development can be actively promoted versus those where it should be controlled to avoid overwhelming the road network. The study concludes that while the method is effective for large-scale networks, future work must incorporate changes in OD distribution patterns to more accurately reflect the dynamic relationship between land use and traffic.

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• traffic density