The Flaw in the Paseo Delicias Roundabouts

Author: Committee in Support of Signals

Date: June 02, 2015

Roundabouts handle intersection traffic effectively in most instances, but not all. The problem comes with intersections subject to heavy peak traffic such as occurs in the morning and afternoon in the Delicias Corridor. This peak traffic can cause imbalance in the roundabout, and that is the fatal flaw. It starts to become a factor when over 50% of the roundabout traffic is concentrated in a single leg, particularly the upstream leg.

For example, in the morning the forecasted peak hour traffic on the Delicias approach to the Via de la Valle roundabout (the upstream leg) is 1,140 vehicles comprising 64% of the traffic addressing that roundabout. With an entry speed set forth in the EIR of 29 mph, and a vehicle entering every 3 seconds, it makes it extremely difficult for the 300 vehicles entering from the Village to merge into the roundabout during each peak hour. A solution for this problem is to put traffic signals in the roundabout, but this, of course, severely impacts vehicle throughput.

The roundabout can be replicated in computer modeling, and this was done in the EIR. However, the Sidra software, the standard at that time, used HCM 2000 methodology, and each leg of the roundabout was addressed independently, and therefore did not reflect the imbalance situation. Furthermore, the software was developed in Australia, and was geared to drivers very familiar with utilizing roundabouts. Accordingly, the projected Level of Service (LOS) was considered to be acceptable.

Recognizing a potentially serious problem with the proposed roundabout project, a group of residents commissioned a new study using the same data as the EIR, but with an updated software package. The new software, based on the latest HCM 2010 methodology, has algorithms that do a better job of recognizing imbalance, and better provides for U.S. drivers. The result was Level of Service F for every Paseo Delicias approach to the 3 roundabouts morning and afternoon. It would be indefensible to spend $6 million and endure 18 months of construction disruption for a project with such a high potential for failure. It should be pointed out that the company performing the new test was recommended by LLG (who did the EIR Study) and approved by the County, and both entities reviewed the results.

Finally, it should be noted that the same data was run for traffic signals using the updated software and they passed with acceptable ratings.

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