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    Concepts Chapter 15/Two-Lane Highways Page 15-6

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    Sk00tkab00t
    The document is a chapter from the Highway Capacity Manual that discusses guidelines and analysis methods for evaluating mobility on two-lane highways. It provides information on factors that affect traffic flow such as lane widths, no-passing zones, and basic relationships between flow rate, average travel speed, and percent time spent following. The chapter also covers capacity and levels of service for two-lane highways, noting the maximum one-directional capacity is 1,700 passenger cars per hour and two-way capacity is 3,200 passenger cars per hour.

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    Concepts Chapter 15/Two-Lane Highways Page 15-6

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    Sk00tkab00t
    34 views1 page
    The document is a chapter from the Highway Capacity Manual that discusses guidelines and analysis methods for evaluating mobility on two-lane highways. It provides information on factors that affect traffic flow such as lane widths, no-passing zones, and basic relationships between flow rate, average travel speed, and percent time spent following. The chapter also covers capacity and levels of service for two-lane highways, noting the maximum one-directional capacity is 1,700 passenger cars per hour and two-way capacity is 3,200 passenger cars per hour.

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    HCM Page 15-5

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    The document is a chapter from the Highway Capacity Manual that discusses guidelines and analysis methods for evaluating mobility on two-lane highways. It provides information on factors that affect traffic flow such as lane widths, no-passing zones, and basic relationships between flow rate, average travel speed, and percent time spent following. The chapter also covers capacity and levels of service for two-lane highways, noting the maximum one-directional capacity is 1,700 passenger cars per hour and two-way capacity is 3,200 passenger cars per hour.

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    34 views1 page

    Concepts Chapter 15/Two-Lane Highways Page 15-6

    Uploaded by

    Sk00tkab00t
    The document is a chapter from the Highway Capacity Manual that discusses guidelines and analysis methods for evaluating mobility on two-lane highways. It provides information on factors that affect traffic flow such as lane widths, no-passing zones, and basic relationships between flow rate, average travel speed, and percent time spent following. The chapter also covers capacity and levels of service for two-lane highways, noting the maximum one-directional capacity is 1,700 passenger cars per hour and two-way capacity is 3,200 passenger cars per hour.

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    of 1

    Highway Capacity Manual: A Guide for Multimodal Mobility Analysis

    Level terrain, and


    No impediments to through traffic (e.g., traffic signals, turning vehicles).
    Traffic can operate ideally only if lanes and shoulders are wide enough not
    to constrain speeds. Lanes narrower than 12 ft and shoulders narrower than 6 ft
    have been shown to reduce speeds.
    The length and frequency of no-passing zones are a result of the roadway
    horizontal and vertical alignment. No-passing zones may be marked by barrier
    centerlines in one or both directions, but any segment with a passing sight
    distance less than 1,000 ft should also be considered as a no-passing zone.
    Passing in the opposing lane of flow may be necessary on a two-lane
    highway. It is the only way to fill gaps forming in front of slow-moving vehicles
    in the traffic stream. Restrictions on the ability to pass significantly increase the
    rate at which platoons form in the traffic stream, since motorists are unable to
    pass slower vehicles in front of them.

    Basic Speed Flow Relationships


    Exhibit 15-2 shows the relationships among flow rate, ATS, and PTSF for an
    extended directional segment of two-lane highway under base conditions. While
    the two directions of flow interact on a two-lane highway (because of passing
    maneuvers), this chapter analyzes each direction separately.
    Exhibit 15-2(b) illustrates a critical characteristic of two-lane highways.
    Relatively low directional volumes create high PTSF values. With only 800 pc/h
    in one direction, PTSF ranges from 60% (with 200 pc/h opposing flow) to almost
    80% (with 1,600 pc/h opposing flow). In contrast, typically acceptable speeds can
    be maintained on uninterrupted-flow multilane highways at relatively high
    proportions of capacity. However, on two-lane highways, service quality begins
    to deteriorate at relatively low demand flows.

    CAPACITY AND LOS


    Capacity
    A two-lane highway under base conditions is 1,700 pc/h in one
    direction, with a limit of 3,200 pc/h for the total of both directions. Because of the
    interactions between directional flows, when a capacity of 1,700 pc/h is reached
    in one direction, the maximum opposing flow is limited to 1,500 pc/h.
    Capacity conditions are rarely observed except in short segments. Because
    service quality deteriorates at relatively low demand flow rates, most two-lane
    highways are upgraded before demand approaches capacity. Nevertheless,
    evaluating two-lane highway operations at capacity is important for evacuation
    planning, special event planning, and assessment of the downstream impacts of
    incident bottlenecks once they are cleared.
    Two-way flow rates as high as 3,400 pc/h can be observed for short segments
    fed by high demands from multiple or multilane facilities. This may occur at
    tunnels or bridges, for example, but such flow rates cannot be expected over
    extended segments.

    Concepts Chapter 15/Two-Lane Highways


    Page 15-6

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