Tidbit Tuesday
Tuesday, July 14, 2020 at 4:21:54 PM
Tidbit Tuesday #14

HCM Analysis versus Field Observations - What to do with Mix-Match Findings

Remember in grade school when we had “mix-match” day during Spirit Week, and we showed up wearing plaid pajama bottoms with a tye-dye shirt? Well, that was great for Spirt Week, but it’s not a good look for Traffic Reports, specifically when the operational analysis doesn’t match the field observations. It’ll probably get you a lot of strange and confused looks. But don’t worry, there’s still hope!

Typically for an unsaturated network, we utilize the Highway Capacity Manual (HCM) methodology to analyze intersections and corridors. The HCM looks at each individual intersection as a standalone location. It does not take into account the effects of other upstream or downstream intersections. You may find yourself studying a corridor where you observed a 3,000’ queue at an intersection that spills back through one or more other intersections. When you get back to the office and analyze this with HCM software tools, the results of your analysis will likely show one intersection failing and all the other intersections as operating without problems. But you know this is false. These other intersections are not actually operating without problems because they’re being impacted by the queue spillback. In this case, the field observation notes should be referenced in the Traffic Report to explain why there is a discrepancy. If a valid justification is provided, DOTD may determine the mix-match findings to be acceptable, and will likely rely on the field observations as the more accurate source of information.

If you'd like to contact us with any comments, questions or tidbits of your own, please email TrafficEngineering@la.gov.

Thanks for reading!

Bonus tip: During Alternative Analysis, it is likely that the conceptual designs will take the problematic intersection out of saturation mode and alleviate the queue spillbacks that occurred during existing conditions. In that case, the HCM outputs for those other intersections could be considered reliable and accurate.