Los Alamos has a modified roundabout at San Ildefonso and Diamond that performs well

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The San Ildefonso roundabout is well designed and operates well in its most important role. The roundabout outperforms the multi-direction stop/yield intersection it replaced. Diamond drive is the 4-lane road connecting with downtown LA.

There are four main peak-traffic streams.

  • AM commuters from Barranca Mesa use the bypass (see van in photo)
  • AM commuters from North Mesa enter from the South (left) and make a 270 degree left turn on the circle
  • PM commuters to Barranca make a 270 degree left to the North (right side of photo)
  • PM commuters to North Mesa use a bypass not visible here to turn onto S. San ildefonso.

Nominal diameter of this roundabout is about 90' (without the bypasses, which are not relevant to the proposed NM502 roundabouts). Proposed roundabouts for the NM502 corridor are currently 110' diameter.

Photo above shows central island with a "hard curb", surrounded by a slightly raised "apron" that facilitates truck circumnavigation. Part of a pedestrian crosswalk is visible in the lower right. Nobody is supposed to visit the island on foot.

Approaching from Barranca Mesa (N. San Ildefonso).
Same entrance, closer. Shows the right-hand turn bypass that makes life good for Barranca-ians heading to work. Notice that the "bowl-shaped" terrain gives excellent previews of all entrances and exits. The bowl also give bicyclists a thrill as they approach the circle!

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