11/02/2026
HCM 7.1 for Freeway Analysis is Here!
The Highway Capacity Manual was updated in December 2025 to incorporate new research on freeway merge, diverge, and weaving segment analysis. The research project developed a new methodology and provided replacement HCM chapters that were subsequently revised and are now available for free on the National Academies website. The new chapters are known as HCM 7.1 to distinguish them from the original 7th edition chapters.
The previous methods for ramp junctions and weaving segments were developed separately so for certain combinations of traffic volumes and roadway geometry, you could get unexpected results. For example, the before condition with a merge segment followed by a diverge segment might show better performance than the after condition where an auxiliary lane is added to form a weaving segment. The new research was based on a much larger sample size, used modern sensor data, and included more ramp configurations than the research used to develop the previous method.
The new methodology starts with determining the performance measures for an equivalent basic freeway segment: that is, a segment without ramps. A speed impedance term due to the merge, diverge, or weave condition is then subtracted from the basic segment speed to determine the analysis segment’s speed. If the ramp volume is zero, then speed impedance is also zero, and the segment performs the same as the equivalent basic segment.
The researchers found that the critical density, the density at capacity, occurred at about 35 passenger cars per lane per mile (pc/ln/mi) in the new data set. This is lower than previous weaving capacity of 43 pc/ln/mi. The previous method did not have a capacity threshold for density for merge and diverge segments. Since capacity represents the threshold between level of service (LOS) E and F, the LOS thresholds were revised as shown in the table below. These new thresholds match those for basic freeway segments for LOS A and B but have narrower ranges for LOS C through E.
The new methods are simpler but use the primary inputs from the previous methods. Merge and diverge segment performance is based on the number of freeway lanes, the mainline and ramp volumes, ramp speed, and the ramp acceleration or deceleration lane length. Weaving segment performance depends on number of freeway lanes, weave lanes, and lane changes; the origin-destination volumes; and the weave type.
Here are some additional notable changes.
- Merge and diverge segments can be as long as the acceleration or deceleration lane even if the length is greater than 1,500 feet.
- The presence of adjacent ramps no longer affects the performance of merge and diverge segments.
- The density for the ramp influence area is no longer calculated. The density for merge and diverge segments applies to all lanes.
If your reviewing agency is not familiar with the HCM 7.1 methods, it’s a great opportunity to let them know.
