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
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,
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.
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
feed rates: US Federal roundabout guidance (NCHRP
672, sec. 188.8.131.52) 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
- 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.
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
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.
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.
- 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.
- 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).