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WIlliams: NM502/Central - Traffic Planning Gone Awry

on June 15, 2016 - 9:00am
Note that both EB and WB NM502 are designed for 40mph. Note the 50-ft scale at the lower right that pertains to reaction distance inside the roundabout, as well as to design parameters. Note, too, that a "fastest path" was not provided by the designer (indicated by a dash in the table) for WB NM502 traffic on the continuation of this state highway. Image Courtesy/Joel WIlliams
Los Alamos
A roundabout will be installed at NM502 and Central because that is what the council has decreed and that is what the NMDOT is going to build for them or do nothing at this stage. At least it will come with needed utility upgrades and dual lanes from DP east past Central. To this extent, it is better than nothing. 
It will be interesting to watch the high speed, straight, dual lanes of PM traffic cross the Canyon/Arroyo intersection and then merge into a single lane in the congested church, swimming pool and Sombrillo area. At least, the current merging occurs as traffic has to slow coming around the "DP curve".
This letter-to-the-editor, however, is intended to express my views (excerpted from my letter today to the NMDOT) about the west-bound AM performance that can be expected of the roundabout once installed.
From this "additional" information, the planned roundabout fails on 3 points!
  1. WB NM502 flow into Central will "NOT be CALMED"! In fact, westbound traffic on NM502 that enters the proposed roundabout at the posted speed of 35mph can carry that speed to a point nearly 500 ft deeper into Central than is carried by the current intersection! I participated in all of the 2011 NM502 Knecht-Tewa "technical evaluation sessions". The clamor of those who wanted roundabouts was the need for "traffic calming, traffic calming, traffic calming". For the Central-NM502 intersection, this primarily means calming the WB NM502 flow into Central. Well, this roundabout does not do "traffic calming", but, in fact, does just the opposite as it deposits high speed traffic much closer to a more congested area (nearer the entrance to the Central Avenue shopping center) than the current intersection even does. In this respect, this planned roundabout is worse than even the current intersection's long feed into the business-part of Central!
  2. WB NM502's continued flow is not smooth through this intersection. Per 18.31.6 NMAC, the NMSHTD provides procedures and standards "to maintainsmooth traffic flow" and "to protect the functional level of state highways". The planned roundabout relegates the traffic movement of WB NM502 to the lowest priority of all of the flows in this roundabout by "forcing"  it go through more than a 90 degrees deviation from straight! Indeed, any traffic entering the roundabout at the designed or posted speed will be unable to slow quickly enough to make the sharp bend to continue on WB NM502; that traffic will either end up going into the Central entry or crashing through the Central splitter island into the Central exit lane. In this respect, this planned roundabout violates the NMDOT manual's directive to provide performance functionality to state routes.
  3. The AM flow delay differences for each of the roundabout WB lanes will put substantially more traffic into Central than current. The 2010 AM (and current) WB flows at the current intersection present a driver with little resistance in choosing either Trinity or Central: 694 choosing Trinity and 549 choosing Central. With the planned roundabout, however, there would be considerable difference in delay times to go to each. The ARCADY model indicates a "B" delay of 10.76s for the planned roundabout's right lane to Central and 4th, while it has a "C" delay of 20.3s for the left lane to Trinity. Based purely on the delay differences, one could expect the current traffic which flows primarily (56%) into Trinity will flow primarily (65%) into Central and 4th with the proposed roundabout. [Think of it as customers moving to the fastest checkout lane of a bank or grocery.] This would add up to 50% more traffic to Central and 4th streets. Would Central be able to handle significantly more flow? Knecht at City Hall is the first street that is not a residential or shopping center roadway. The proposed roundabout's sharp detouring action will stifle flow to Trinity (NM502) by up to 40%. Why would the state choose to install a design that would severely restrict flow on its own highway when it makes special efforts to provide better flow?
 Originally, NMDOT preferred a T-signalized intersection that would have operated in a completely opposite manner than this planned roundabout for WB NM502: 
1) Priority would be given to WB NM502 to Trinity (the continuation of WB NM502,) while;
2) Real traffic calming and easing of flow volume into Central would be provided by the right-turn at the signal - near the current 4th St entrance from Trinity - with the subsequent left-bending of the flow into Central. 4th would form a T-intersection with Central, instead of the current cross. (T-intersections are safer that cross ones. So, why is Canyon being made a cross intersection with Arroyo lane?)
The planned roundabout at Central-NM502 has serious flaws that have evolved since the original roundabout concept was proposed by the Los Alamos Public Works Department in 2011 to the "technical advisory committee" under the auspices that a roundabout would be "traffic calming" while still providing smooth flow for NM502.
It was under this auspices that the Los Alamos County Council made its 2012 request to the NMDOT to have a roundabout installed. Clearly, this auspices NO LONGER applies with the planned roundabout. The only feature that seems to have continued credibility is putting a "piece-de-art" in the island. 
I would hope that NMDOT would have the temerity to recognize that this roundabout design is NOT the solution to traffic issues at Central-NM502 that was originally sought by and presented to the Los Alamos County Councilors and citizens and will proceed with the only option that NMDOT can easily and quickly substitute in its place: NMDOT's originally preferred, signalized, T-intersection that gives flow priority to NM502 (a state highway, no less!), while providing desired traffic moderation into Central, as was the original desire of the LA county's technical advisory committee that studied the issue.
So why isn't the original signalized T-intersection going to be built? Because a very few (our elected) can see that it is otherwise. Politics.