Two-Lane Highways analysis – An overview of HCM 7th Edition methods

Two-lane highways are a critical component of the US transportation system, providing connections between rural and urban areas and with the Interstate Highway System. ?

The Highway Capacity Manual 7th Edition (HCM7) includes a new methodological framework to analyze the quality of service of Two-Lane Highways, which is based on the research project NCHRP 17-65 – Improved Analysis of Two-Lane Highway Capacity and Operational Performance.

This article will highlight the major changes in the HCM7 methodology compared to the previous HCM6 methods.

New Service Measure – Follower Density

The HCM7 methodology introduces a new service measure called “Follower Density”. It is defined as “the number of vehicles in a follower state per mile per lane”. In this context, a “follower” vehicle is defined by a headway equal to or less than 2.5s.

Another change to the methodology is the removal of highway classifications (Class I, Class II and Class III), which would determine the appropriate service measures per the HCM6 methodology. With the new methods, Follower Density will always be the service measure for all two-lane highway configurations.

This change overcame issues with the Percent Time Spent Following (PTSF) performance measure in 2016, which was difficult to measure directly in the field.

HCM Exhibit 15-6 – New LOS Criteria for Two-Lane Highways

Other performance measures provided by the HCM7 methodology are:

• Average Speed: average spot speed at the endpoint of the segment
• Percent Followers: count of followers vehicles divided by the total number of vehicles

Facility Analysis and Segmentation

The HCM7 methodology allows the analysis of three different segment types, which can also be aggregated into a facility-level analysis:

• Passing Zone: passing is permitted
• Passing Constrained: passing is prohibited
• Passing Lane: a second lane is added to a given direction of travel, allowing vehicles to pass without going into the opposite direction lane

Two-Lane Highways Segmentation Framework

The HCM6 methodology did not offer this feature, which required the user to provide the % of no-passing zones and the length of any passing lanes within the segment.

Sensitivity to Horizontal Curvature

The HCM7 methodology can now address horizontal curvature elements for more accurate estimations of speed. A segment can be divided into sub-segments which can be tangent or curve – for the latter, different horizontal curve classification groups are established based on curve radius (ft) and superelevation (%).

Sample Horizontal Curve Inputs

Free-Flow Speed (FFS) Estimation based on Posted Speed Limit

The HCM7 methodology redesigned the equations to estimate free-flow speed when field measurements are not available. This approach is consistent with other Uninterrupted Flow methods in the HCM (freeways and multilane highways).

Analysis of 2+1 (Super 2) Sections

A Super 2 Highway, also called “2+1 Highway”, is a two-lane highway configuration with a continuous three-lane cross-section, with the middle lane being a passing lane that alternates direction. The HCM7 methodology also includes equations to measure the performance of this highway type.

Super 2 analysis checkbox in the HCS user interface

HCS Implementation

The new Two-Lane Highways methodology is fully implemented in HCS2022, already available to all HCS users with an active subscription.

Two-Lane Highways -?Facility analysis HCS interface