HCM 2010 Update – Stop Sign & Roundabout Webinar

Here are the slides from yesterday's webinar (Download Unsignalized HCM 2010 Presentation).  Highlights from the discussion –

Roundabouts

• This whole chapter is new.  We can finally compute Level of Service for roundabouts!
• Capacities at roundabouts in the U.S. are currently much lower than capacities observed in other countries.
• Calculations are based on simple models.  Experience in the U.S. shows roundabout performance is largely impacted by the variability in driver behavior, not specific geometrics configurations.
• Not included in the Level of Service calculations – geometric configurations (other than number/type of lanes), oversaturated conditions, or interaction effects of upstream/downstream intersections.  Other models should be used if these items are of concern.
• Delay thresholds are the same as those for stop sign controlled intersections, which are lower than the thresholds for traffic signals.
• On multi-lane approaches – the volumes in the two lanes are not assumed to be the same.
• There are calibration tools, which can ideally be tweaked based on local driving behavior data (this is true of the entire HCM 2010).

All Way Stop Sign Control

• Added methodology to model three lanes of approach instead of just two.
• New queuing model that is similar to the side street stop sign control method.

Side Street Stop Sign Control

• Added methodology to model six lane major roads instead of just two and four lane roads.
• Added methodology for modeling u-turns (our traffic counters will have more to keep track of).
• More refined method for dealing with the impact from upstream/downstream intersections.
• Better modeling of impacts of short left turn lane storage and large radii for right turn movements.