Caltrans has had a longstanding preference of using the Leisch method nomograph for analyzing freeway weaving segments over the HCM methods. This preference has existed for so long though I wander if it is still a valid argument? HCM has been through many versions in the nearly 20 years I've worked here, has there been a significant change to HCM's method of analyzing weaving segments in that time? say, since HCM 97, or 2000?

Yes, the procedure was substantially rewritten for the HCM 2010 using lane changing rates, including a dynamic maximum length equation and incorporating the capacity and speed adjustments for weather, driver familiarity and lane closures now in all segment analyses. Please let me know if you want more information about the new methodology.

Oregon used the Leisch weaving method until recently, when they adopted the current HCM method. One reason Oregon had used the Leisch method was that it didn't limit the weaving length to 2,500 feet, which the HCM2000 method did. However, NCHRP Project 03-75, which developed the current HCM method, noted in its final report that the Leisch method extrapolated weaving lengths far beyond the limits of its database, so it's not known how good its estimates were for longer weaving lengths.

Thank you both. I'll take a look at the HCM 6 method.