One of the more significant additions to the new 6th Edition of the Highway Capacity Manual (HCM) are equations to account for the effects of work zones. Based on NCHRP Report 03-107, reduction factors have been developed to reduce capacity, free flow speed, and saturation flow rates in four of the analysis modules.
Several state Departments of Transportation have their own procedures related to analyzing different work zone schemes, but the new HCM methodology significantly improves our ability to analyze the potential impacts of different work zone alternatives. The formulas for the reduction factors are simple to use if you have the following inputs at hand for each type of factor.
To adjust the capacity and free flow speeds utilized for freeway analyses, you’ll need the following inputs:
- Barrier Type. Either concrete or cone/type II barricade/plastic drum
- Area Type. Urban or rural
- Lateral Distance. Distance from the edge of the travel lane to the barrier (in feet from 0 to 12)
- Time of Day. Daylight or nighttime
- Speed Limits. Normal speed limit and work zone speed limit
- Number of Ramps. The number of on-ramps and off-ramps within three miles upstream and downstream of the work- zone area
- Number of Lanes. Number of total lanes and the number of lanes that will remain open within the work zone
- Queue Discharge. Pre-breakdown capacity drop due to queuing conditions (defaults are provided if this isn’t measured in the field)
To adjust the free flow speeds for Urban Street Segments is simpler than the adjustments for analyzing work zone effects on Freeways. The adjustments for Urban Street Segments only require you to know:
- The length of the work area
- The posted speed limit within the work zone
To adjust the saturation flow rates at Signalized Intersections, you’ll need to know:
- The number of lanes open through the work zone segment
- The number of lanes open on each intersection approach in the work zone
- The lane widths in the work zone on each intersection approach
- The distance between the stop line and the nearest point of the work zone along the segment
The last adjustment factor is for Two Lane Highways where one of the lanes is closed for the work zone. This ends up being a hybrid analysis in that the single lane will operate like a signal controlled intersection with a flagger letting one direction move through the work zone at a time. To make the capacity reduction, you’ll need to know:
- The length of the work zone
- An estimated “signal green time” for the flagger operation
It doesn’t look like the Highway Capacity Software or any other software package currently implements this new work zone methodology. It would take less than a day to setup a spreadsheet to develop the reduction factors that you could then apply within the analysis software you use.
We haven’t had the opportunity to analyze work zones for a client yet, so we haven’t put together a spreadsheet to do the calculations. Let me know if you’ve implemented the work zone methodology from the 6th Edition of the HCM. It would be great to share a case study on the blog.
Also, let me know if you’d find value in a spreadsheet that calculates the work zone reduction factors. We’re thinking about putting together this spreadsheet. Let us know if you would find this type of spreadsheet useful. Email us at Newsletter@MikeOnTraffic.com.