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