NM502 Roundabout design critique

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Joel Williams and William Mead offered assessments and questions re. the planned Trinity/Central roundabout at the Transportation Board meeting on 3/3/16. Articles reporting on the meeting were published in the Los Alamos Monitor 3/6/16 and 3/9/16.

Here is the "95%-complete" drawing together with critique, adapted from

  • William Mead's presentation to the Los Alamos Transportation Board (download presentation slides here). In his talk, Mead referred to a two-lane roundabout in Venice, FL that has a very high accident rate (see references and links here).
  • Joel Williams' article published in the Los Alamos Daily Post, 3/15/16.

Design and Functional Issues

The following list provides some specific comments and links for each item flagged on the design drawing. Many of the items mentioned here have roots in the generic characteristics and problems of two-lane roundabouts.

Dysfunctional gateway: Using a roundabout, especially a 2-lane or highly customized roundabout, as a gateway has drawbacks

  • Visitors to the community will sometimes (often?) be confused; roundabouts differ at different locations.
  • Local users will be surprised when visiting drivers do not follow the usual, local rules of the road.
  • The central island offers some landscaping opportunity, but visitors to Los Alamos will mostly be concentrating on navigation and competing traffic.

Uneven feed rates: US Federal roundabout guidance (NCHRP 672, sec. 2.2.3.2) notes [italics added] that

Roundabouts tend to treat all movements at an intersection equally, with no priority provided to major movements over minor movements. Each approach is required to yield to circulating traffic, regardless of whether the approach is a local street or major arterial. This may result in more delay to the major movements than might otherwise be desired. This problem is most acute at the intersection of high-volume major streets with low- to medium-volume minor streets (e.g., major arterial streets with minor collectors or local streets). Therefore, the overall street classification system and hierarchy should be considered before selecting a roundabout (or stop-controlled) intersection. This limitation should be specifically considered on emergency response routes in comparison with other intersection types and control.

For the planned roundabout, two of the entries carry dominant flows that will lead to local aberrations in yield practices. Drivers from the dominant flow directions tend not to yield as much as the official roundabout protocols, while those from the minor flows tend to yield more often than required.

  • During the morning rush, the westbound flow entering from East Road is large (red path arrows).
  • During the afternoon rush, the eastbound flow from Trinity Drive dominates (orange arrows).
  • At either time, vehicles entering from eastbound Central heading for eastbound Trinity (purple path) will have difficulty. This willl be worst in the evening rush, when these vehicles will be facing heavy traffic that should yield but might not.
Widely varying deflections lead to different speeds at the various exits. One goal of this project was to reduce speeds of westbound vehicles turning onto Central Avenue while maintaining speeds onto Diamond Drive. As designed, the reverse will occur: low deflection for traffic onto Central will permit high exit speeds, while the large deflection for the westbound flow onto Trinity will reduce speeds, an undesirable inhibition of a large traffic flow.
Dangerous for pedestrians, bicycles, and motorcycles: 2-lane roundabouts are well-known, serious hazards for users other than cars and trucks. For cars and trucks, 2-lane roundabouts can also increase the accident rate. The cause is obvious: drivers who are concentrating on navigation and avoiding collisions with other vehicles do not have much attention available to devote to pedestrians. Safety for vehicles is addressed on the next page.
Many built-in conflicts and dependence on users' yield behavior: In a signalized intersection, spatial conflicts are mitigated by separating flow times. In a roundabout, the flows are random, and avoiding collisions is dependent on users' real-time choices. When the roundabout is lightly used, these conflicts can easily be avoided, but as the traffic flow becomes heavy, collisions are more likely. Note that the solid arrowheads in the drawing indicate yield points, and there are many.

Traffic conflicts between Central Ave. eastbound and Trinity eastbound during the evening peak. Another project goal is to improve access to NM502 for drivers from Central. Stop-and-go traffic is often seen here with the existing roadway. Peak-time-service for the unfortunate souls who wish to turn onto NM502 from Central Ave. is very likely to worsen, if the planned roundabout is built. Drivers will be confused about right-of-way, especially those unfamiliar with the roundabout.

Evening rush analysis

  • During the evening peak, eastbound Trinity (orange path arrows, above) has heavy traffic.
  • The "official" FHWA rules of the road would require traffic entering the roundabout from both lanes of eastbound Trinity Dr. to yield to traffic that has already entered the roundabout from Central Avenue eastbound (purple path arrows). Thus, if Central Ave. drivers assert their right-of-way, the Trinity eastbound traffic would be frequently interrupted. Based on experience with the San Ildefonso roundabout, this is unlikely to happen.
  • Roundabouts develop "unofficial", local rules of the road; faster, higher-volume traffic tends to override "official" rules of the road.
  • Consequently, it is more likely that vehicles from Central already in the roundabout will probably have a difficult time finding a gap to enter the eastbound traffic from Trinity. If a few cars get trapped in the roundabout, westbound NM502 traffic could be cut off.
  • Apart from whether "official" or "local" rules apply, this roundabout shares with most 2-lane roundabouts a dangerous conflict that can occur as (eastbound NM502) traffic exits. The inner lane is marked for through or left turn traffic. The outer lane is marked for through-traffic only. If somebody gets into the outer lane by mistake, and, in panic, turns left at the exit, he might well hit a car that is trying to go straight through. This makes traveling in the inner lane to go straight a particularly risky choice. On the other hand, if many drivers choose not to use the inner lane to go straight, then traffic mostly proceeds in the outer lane, and throughput will suffer.

Morning rush analysis

  • In the morning peak (westbound NM502 at high volume), vehicles already in the roundabout from East Road (red path arrow, above) will force vehicles entering the roundabout from Central (purple path arrow) to wait for a gap.
  • Since the roundabout slows traffic to ~25 mph, spacing of traffic will be reduced, making gaps hard to find.
  • There is little or no left-turning morning traffic from eastbound Trinity that might cut-off the high westbound flow to create a gap that would help Central users enter the roundabout. Even if there were occasional vehicles eastbound on Trinity that tried to turn left, those vehicles would probably not be able to assert their right of way (as described in the evening analysis, above).

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