Ada County Highway District

ACHD's Road Wizard Sunday, February 16, 2020 ACHD's Road Wizard

What is a bridge? Roundabouts

The Road Wizard Replies

Dear Road Wizard: Help me settle a bet with my husband. What does ACHD consider to be a bridge? He says a bridge must transport people; I don't agree. So, what is ACHD's stance on the matter of bridges?


Road Wizard:

Even without knowing the high stakes involved with this bet, the answer is pretty straightforward. A bridge is a structure that spans larger than 20 feet, according to federal guidelines - anything under 20 feet is considered a culvert.

Although the term culvert/bridge can make a big difference for planning purposes, ACHD made the decision more than two decades ago to treat culverts in the same manner as bridges when it comes to inspections and maintenance so that the different spans get the same treatments. It is also important to note that many culverts will still have typical bridge structures such as footings, abutments, main beams, and the use of concrete decks, among other things.

So, in short, you win the bet. Bridges, according to ACHD, don't need to solely transport people in order to be called a bridge.

Dear Road Wizard: I hate roundabouts. I wish ACHD would stop putting them in everywhere. They are not better and frankly seem like accidents waiting to happen. This is not Europe.


Road Wizard:

As of the end of 2013, there were over 3,000 roundabouts throughout the United States, with at least one in each of the fifty states. The first in the Treasury Valley was installed in Nampa at the intersection of Amity and Happy Valley. According to many studies that have been conducted over the past ten to fifteen years, roundabouts offer several benefits to drivers and other roadway users:

Safety - Studies have shown that single lane roundabouts typically have an overall crash rate much lower than that of comparable signalized intersections, while crash rates for multi-lane roundabouts are comparable to signals. However, severe crashes are significantly reduced at both single lane and multi-lane roundabouts compared to signalized intersections. This is largely due to the roundabout design that encourages drivers to slow down when entering the circulating roadway.

Traffic operations - Again, many studies from across the U.S. have found roundabouts to be more efficient in moving vehicles through intersections than all-way stop-controlled locations. They also have similar capacities as signalized intersections with the same number of lanes, but over the course of a day, many vehicles who would be stopped at a red light with no cross traffic could pass through a roundabout with little to no delay.

Environment - Roundabouts decrease vehicular emissions by about 15-25 percent compared to signals. They also reduce delay, which improves vehicular fuel economy.

Cost - Roundabouts have similar upfront costs compared to signalized intersections, though this does vary on a case-by-case basis. However, the long-term maintenance costs are less for a roundabout than a signal, as there are fewer electrical components that need periodic replacement.

While these are all benefits associated with roundabouts, they are still fairly new to Ada County, which can be confusing to drivers who are unfamiliar with the operation. Drivers approaching a roundabout are required to yield to crossing pedestrians as well as vehicles within the circulating roadway. There is information in the Idaho Drivers Manual on roundabouts, which is helpful for new drivers. As we see more roundabouts in the area, I expect residents will become more familiar with their operations and welcome them for the reasons listed above. They sure make my broom travel easier!

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