section widths more conducive to non-auto modes and placemaking strategies. Large, widely-spaced thoroughfares, fed by disconnected local and collector streets, have the opposite effect. They channel traffic onto a few major routes, limit alternatives in the event of an incident or emergency, and create congestion and delay at major intersections. Trip lengths tend to be longer and less direct, and the lack of local street connectivity and large cross section widths impede the use of non-auto modes. An ideal network concept places arterial networks in a grid pattern of continuous 4-lane roadways at a general spacing of one-half mile (Figure 45). Shorter spacings of one-quarter mile can be accommodated in dense urban areas. The idealized grid offers numerous benefits for a robust multimodal network. The ½-mile spacing of signalized intersections on major arterials provides more efficient traffic progression in response to peak and off-peak traffic conditions. This spacing creates a 640 acre “cell” for development, with 160 acre “subcells” where streets can be designed to tame traffic and create a safe, livable environment. Local bus service on major streets places residents within a reasonable (¼-mile) walking distance of a bus line. Frequent placement of through routes helps to avoid the need for wide six-lane roadways with multiple turn lanes at intersections. Four-lane roadways are easier to integrate into neighborhoods than wider roadways and complement urban placemaking and complete streets concepts. On more heavily travelled routes and where wider cross sections are needed, alternative intersection designs (e.g., Michigan U) can be employed to reduce pedestrian crossing widths and allow two-phase signals by eliminating direct left turns. Levinson (2000) offers detailed analysis and guidelines. Pedestrian crossing needs can be accommodated through flexible location of signalized pedestrian crossings.
Figure 45. Illustration of ideal arterial network spacing. Source: TRB Access Management Manual, 2014.
A process for accomplishing more robust network spacing in the context of a thoroughfare plan is illustrated in NCHRP Report 917: Right-Sizing Transportation Investments: A Guidebook for Planning and Programming (Duncan et al., 2019). It is summarized briefly below in text and illustrated in Figure 46. • Step 1: Overlay ideal grid per spacing guidelines and compare with horizon-year network plans. • Step 2: Adapt ideal grid to existing roadways and environmental features.
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