NM502 with roundabouts: capacity, congestion, and efficiency for motor vehicles will worsen

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The roundabouts planned for Tinity/East are 136' nominal diameter (as of 6/24/11) compared with San Ildefonso's roundabout at 90' (110' if you include the bypasses). Note that increasing the roundabout diameter increases its capacity only gradually.

Holding the centripetal acceleration constant, comfortable speed range on roundabouts currently proposed would be: 18-23 mph.

Let's compare capacity to demand for the existing roundabout and two locations along the NM502 corridor. The table below neglects possible "slip lanes" or bypasses, which have not been discussed in detail in the Study. My current view is that adding a bypass in any of the proposed roundabout locations would be insignificant. To review the "Simple Model", click here.

Circulating speed (mph)
Peak demand (veh/hr)
Capacity (simple model)
Utilization (simple model)
Diamond at San Ildefonso (existing)
Trinity Dr. at Ashley Pond
East Rd. at airport

Remember that the simple model discussed here is only a rough guide. The conclusion that is probably durable is that the single lane roundabouts proposed for NM502 would be significantly more congested than the roundabout at San Ildefonso.

Now, recall the empirical performance prediction available from NCHRP672. Using the nominal capacity of 1130 veh/hr, with little or no cross traffic, we see that San Ildefonso is operating at a utilization approaching 100%. A similar roundabout on Trinity Drive or East Road would be operating at greater than 100% volume-to-capacity ratio. In this range of operation, the predictions are probably not very reliable. A roundabout operating near or above its capacity can easily "choke", i.e., large queues and stop-and-go traffic can decrease the throughput below that estimated for a working roundabout at 85% of capacity or less.

We can apply the delay estimate from NCHRP672 to see that the delay would be greater than 40 seconds. A string of 9 roundabouts, as currently proposed, would probably not cause 9 times as much delay, since the first roundabout in the string (from either direction) would decrease the traffic volume traveling to the next roundabouts in line so that they could function better than the first encountered. Large queues would accumulate at the end roundabouts.

Joel Williams has performed SIDRA calculations for the proposed roundabouts, and his results also show the roundabouts proposed for NM502 to be near or above capacity, with long delays and queues, and low throughput.

Considering the simple analysis, the NCHRP672 guidelines, or Williams' SIDRA calculations, one obtains similar predictions for the consequences of installing roundabouts on the NM502 corridor:

Estimated utilizations would be higher than for the existing San Ildefonso roundabout

Effect of installing a roundabout on Trinity Drive or East Road: congestion and queues would become worse

Note that predicted conditions take no account of fluctuations in day-to-day loads, adverse driving conditions, and growth of the community or use of the corridor. These factors would further decrease performance. The proposed changes do not leave enough operating margin.

Effect of reducing Trinity to two lanes would be similar to installing a roundabout : lower capacity, more congestion

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