Asymmetrical Roundabouts

[colinstu]

Based on that lane usage, there is no reason to have the second circulating lane at that point. It probably hurts more than it helps.

Which is exactly why that's a terribly designed roundabout. Looking at the way lines are painted on the ground from the aerial, what I illustrated is legitimate. To go East through that roundabout, one would have to stay in the right lane. The left lane should only be used for 1) Going north and ending up in the left lane 2) performing a U-turn and ending up in the left lane 3) performing a right-turn by going around the entire roundabout.

On the North-west quadrant of the roundabout there are lines on the ground that make the right lane a right-only. No such lines exist on the South-west quadrant, which allow drivers to go straight (also no hatched area on the ground).

[johndoe]

Looking at the way lines are painted on the ground from the aerial, what I illustrated is legitimate.

No; your drawing shows the EB drivers crossing the pavement marking once they're in the right circulating lane.  Correct me if I'm wrong, but you should never cross a longitudinal pavement marking once you get into the correct circulating lane of a multilane roundabout.

[DaBigE]

Looking at the way lines are painted on the ground from the aerial, what I illustrated is legitimate.

No; your drawing shows the EB drivers crossing the pavement marking once they're in the right circulating lane.  Correct me if I'm wrong, but you should never cross a longitudinal pavement marking once you get into the correct circulating lane of a multilane roundabout.

Exactly correct. The only reason that segment is dashed is for NB entering traffic to cross. If it was solid, it would indicate that lane (in this case, entry) was for right turns only (similar to the outside lane on the SB approach.

Which is exactly why that's a terribly designed roundabout.

Also correct.

[johndoe]

Exactly. This is why I am glad Wisconsin has a tiered roundabout design program. Newbies (Level 1) cannot design more than a basic single-lane roundabout. Level 3s work the most complex.

How does the state do traffic projections?  Is that done by DOT employees in some sort of planning department, and then analyzed for lane requirements etc?   Or are traffic consultants usually used?

[DaBigE]

How does the state do traffic projections?  Is that done by DOT employees in some sort of planning department, and then analyzed for lane requirements etc?   Or are traffic consultants usually used?

There is a bureau of WisDOT that handles nearly all of traffic projections on the state system. Unless the roundabout is being designed in-house, the consultant then comes up with the lane configuration or configuration alternatives, through software* analysis, based on the numbers provided by the DOT. The draft layout has to get approvals from the WisDOT Regional folks before proceeding beyond 30% plans. There is a nice flowchart (figure 5.1) which depicts the overall design process on page 11 (link opens a pdf from the design manual).

*Until the past year or so, WisDOT primarily used RODEL. With the recent update to the design manual (FDM), they are now going with the HCM method initially, but at the same time acknowledging that other software may be necessary for refinement of more complex situations (due to inherit limitations of the HCM methods).