Congestion depends on the relationship between travel demand and infrastructure capacity. Congestion is most serious at peak commute hours on major roads that serve a wide travel area and tends to worsen as the areas served by the major roads expand. Traffic planners for new communities thus try to anticipate the eventual extent of development and where and how people will travel. Because the plan here prescribes limits on the extent of major LLM roads and directs flows towards the center of town, it may facilitate planning street capacity for maximum and average daily traffic flows.
The LLM network directly connects the residential areas with neighborhood nodes and the center of town. There are no cross-links within or between major branches. For the purpose of planning street capacity, it probably is reasonable to assume that households will travel down the branch to the neighborhood node or town center and then back. The traffic volume along a main LLM branch will be determined by the extent of the minor branches feeding into the main branch (see Figure 24.2), and by the housing density along minor branches. The extent of the minor branches is limited ultimately by the requirement that the travel time from the end of the outer LLM branches to the center of town not be significantly greater than it would be in a conventional street system (otherwise, people might prefer a conventional street system). It is hypothesized that a town radius of 3-5 km is the upper limit on desirable town size.
Thus, in planning an LLM street system, balance between costs (money and loss of land) and benefits (faster and safer travel) can be found relatively easily in choosing street width and speed limits. In general, streets will be very narrow at the ends of residential areas (say, about ~3.7 m ), wider along the radial arms, and widest (about 7.6 m) on the LLM ring road, which will have two relatively wide lanes for motorized LLMs, a completely separate paved path for non-motorized LLMs, and an unimproved pedestrian path. Roundabouts at the major intersections will allow the high traffic volumes near the town center to flow smoothly and safely.
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