9.1.2 Facility Definitions
9.1.2.1 Freeways
A freeway has two or more lanes in each direction access-controlled via ramps. Freeway facilities are typically broken up by “sections” or “segments.” Sections of a freeway are defined as the areas between two ramp gore points. The HCM defines several segment types, including basic, weaving, merge, and diverge segments. See for an illustration of a merge, diverge, and weaving segment. Freeway facilities are typically composed of more segments than sections. Analyzing freeway facilities by sections, by segments, and in its entirety are covered in this chapter.
Segment Type | Sketch-level or macroscopic | HCM-based (mesoscopic) | Microsimulation (microscopic) |
|---|---|---|---|
Freeways (Basic)1 | Travel Demand Model output, Service volume tables | HCS, Freeval, HCM-CALC | Vissim, CORSIM |
Multi-lane Highways (Basic)2 | Travel Demand Model output, Service volume tables | HCS, Freeval, HCM-CALC | Vissim, CORSIM |
Weaving3 | Travel Demand Model output | HCS, Freeval, HCM-CALC | Vissim, CORSIM |
Merge and Diverge4 | Travel Demand Model output | HCM method, HCS, Freeval, HCM-CALC | Vissim, CORSIM |
Two-lane Highways | Travel Demand Model output, Service volume tables | HCS, Freeval, HCM-CALC | Vissim, CORSIM |
Arterial Facilities | Travel Demand Model output, Service volume tables | Synchro, HCS, Freeval, HCM-CALC | Vissim |
Frontage Roads | Synchro or other HCMbased tool | TTI Procedures5 | Vissim |
1
A segment of freeway outside the influence area of any merge (1,500 feet), diverge (1,500 feet), weaving segments (500 feet), or any signalized intersections (2 miles). 2
A segment of highway outside the influence area of any merge (1,500 feet), diverge (1,500 feet), weaving segments (500 feet), or any signalized intersections (2 miles). 3
The crossing of two or more traffic streams traveling in the same direction along a significant length of highway without the aid of traffic control devices. Often formed when merge segments are followed by diverge segments. 4
Primarily occurs at on-ramp and off-ramp junctions with a freeway mainlane. 5
TTI procedures are deterministic but are not HCM-based (mesoscopic). These procedures are discussed in section 3 of this chapter.
Figure 9-1: Illustration of Merge, Diverge, and Weaving Segments from the HCS7 User Guide (Segment Length subsection)
9.1.2.2 Highways
Two types of highways are covered in this chapter: multi-lane (two or more lanes in each direction) and two-lane highways (one lane in each direction). Multi-lane highway segments are analyzed similarly to basic freeway segments. In some instances, multi-lane highways are like freeway segments or arterials, based on the access density and speed.
For two-lane highways, vehicles pass using the opposing lane of traffic. Two-lane highways can include intercity connectors, regional connectors, intracity connectors and may pass through scenic areas or areas with challenging terrain.
9.1.2.3 Arterials
Arterial/urban street segments are bounded by intersections or ramp terminals where the through-movement for the segment is signalized, stop-controlled, or yield-controlled. If these boundary intersection control types do not exist, the segment can be bounded by a two-way stop controlled (TWSC) intersection with stop control on the cross street. Arterial facilities made up of multiple arterial segments can also be analyzed.
9.1.2.4 Frontage Roads
Frontage roads, or service roads, run parallel to a controlled-access facility, serving as connectors between ramps of the higher-speed facility and intersections of lower-speed local streets. Their primary purpose is to provide access to local properties and allow the mainlane users to bypass intersections and driveways. They can either have one-way or two-way operation. Frontage road segments are typically bound by ramps, signalized intersections, or intersections with stop or yield control.