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Williams: Numbers Don't Lie - What Do They Tell Us About Proposed Central-N.M. 502 Roundabout?

on March 14, 2016 - 4:47pm

Image by Joel M. Williams

Los Alamos

Several days ago, I showed what the simple roundabout at Central-N.M. 502 proposed by Los Alamos County Public Works and selected by Los Alamos County Council in 2012 has "morphed" into ... a land-gobbling mammoth. It seems appropriate now to envision how this roundabout will operate.

In order to do this, we need some numbers. Together in the accompanying figure are the current intersection, the roadway of the proposed roundabout and a table of traffic flow numbers during peak hours (morning, noon, and evening), as reported in LAC's DOC(36).pdf.

I have added colored markings on the proposed roundabout roadway to show how the flows will interact during peak rush hours. Feeds of major interest correspond to w-bound N.M. 502 (1), Central Avenue (2), and eastbound N.M. 502 (3). Areas of interest through which these flow are A, B, C and D. Traffic halts at positions 1, 2, and 3 until a break in the flow in a preceding area (traffic moves counter clockwise) allows vehicles to enter.

Sounds simple, but not when flows are massive in one direction or the other as they are along N.M. 502 during rush hours and flow in one of the areas is zero, as it is through area A during those rush hours. (Note all those zeros for eastbound traffic turning "Left" during peak periods). What does "zero Left flow" through area A mean? It means that westbound N.M. 502 has a green light, "free-flow" condition into and through the roundabout during all peak flow hours!

Westbound N.M. 502 now becomes unabated, first into the roundabout during all rush periods. Entry by Central vehicles at point 2 will be no better off than they are now with a simple stop at this intersection. In fact, the divergent, extreme speed-reducing path of the proposed roundabout for westbound N.M. 502 traffic will insure that westbound N.M. 502 is even more compact than it is now, leaving fewer gaps lengths for Central traffic to enter and go eastbound. Thus, this roundabout does NOT solve the morning problem for Central. Lesser, but significant, blockage of Central wanting to go eastbound will occur at noon and evening, as Central is only able to move into area C during breaks in the unrestrained, westbound N.M. 502 flow.

The feed for eastbound N.M. 502 at point 3 are dual lanes from DP (indicated by the light yellow-green path in the roundabout figure). Dual lanes eastbound were stated as the needed fix for N.M. 502 by NMDOT back in 2007. NMDOT actually wanted to run the dual eastbound lanes to the Los Alamos Airport to effect a smooth movement of traffic out of town. The dual eastbound lanes will pass through roundabout area D, but will now be truncated just past the current Canyon Road into a single lane.

While traffic in area C has the "right-away", once the N.M. 502 dual eastbound flow begins, it could fill area D and the dual lanes to the merge point - about 1,000 feet up the road. Central vehicles will have to merge into this traffic in area 3C. It may not be a clean maneuver any more than it is now when friendly folks on eastbound N.M. 502 give a "come-on-in" welcome to waiting Central traffic. Of course, the heavy, eastbound N.M. 502 traffic, in order to get out of town, could just charge into the roundabout without regard to the right-of-way that vehicles in area C have whether the duals are filled to the merge point or not! Traffic from Central would then be unable to proceed across the westbound path to fill area C as it would already be filled. 

The purpose of a redo of the Central-N.M. 502 intersection, beyond including dual eastbound lanes, is to facilitate getting Central traffic onto eastbound N.M. 502. So, how well is this monstrous roundabout going to perform operation-wise to achieve this purpose? NOT at all in the morning. It will only help Central go eastbound in the evening and maybe not as well as planned then. A lot of money and a very deviant flow of westbound N.M. 502 for half a fix. A total fix is available with a signal, however, as NMDOT planned in 2007, but which Los Alamos County Council nixed in 2012 in favor of asking for a roundabout that will be "half" of a solution.

I am guessing that, since the roadway and other associated parts to the project are in solid design shape up to the east and west N.M. 502 sides of the roundabout, subbing in a simple signal intersection where Central currently exits into N.M. 502 (where those heavy tire marks and the faux signal are in the figure) could easily be effected in less than a month and still have the project meet schedule!

This intersection can be fixed without great restructuring and certainly without the great cost that will occur with the current design. The project has seen a 2x-plus escalation in cost since the original, 2007 NMDOT plan. The cost of this roundabout is a frivolous expenditure of somebody else's money now, as LAC is a fixed $1.5M participator! This intersection does not need major surgery. That is why NMDOT chose a signal over a roundabout when it started back into 2007. Little has changed to justify not going with their original choice of a signalized "fix"; I propose that a simple signalized intersection be located at the current intersection (indicated on the figure by the faux signal). This saves the triangle park and there is no need to chop into Los Alamos Public Schools land.

An important additional consideration is the merging of the now truncated eastbound dual N.M. 502. No control of flow to, and thus at, the merge point is available with a cast-in-concrete roundabout; whereas, some flow control over traffic CAN be effected with a signal at Central! Now that a signal is at Smiths, a coordinated signal at Central makes even more sense.

It is time for the Los Alamos County Council and manager to acknowledge that this roundabout is not what they were lead to believe they were buying in 2012. They need to ask NMDOT to sub in a simple, signalized intersection that will do a "full-task, flow-control" at Central, as NMDOT proposed back in 2007 and not have them tear up just about everything possible around that intersection.