Are new roundabouts causing more accidents at intersections in Idaho’s Ada County?
True or false: Roundabouts cause more car accidents than traditional traffic signal intersections.
The answer depends on how many lanes are in the intersection.
According to traffic data from the Ada County Highway District, the county’s first and only dual-lane roundabout at Eagle and Amity roads has seen an increase in accident rates since it was changed from a traffic signal intersection first to a single-lane roundabout, and finally to a dual-lane roundabout in late 2021.
Roundabout locations are decided based on current and future projected traffic volumes. The county highway district conducts traffic studies on individual intersections to determine whether roundabouts or traffic signals would be most effective for traffic.
The highway district cites several benefits to building roundabouts at street intersections.
The circular design of the roundabout “forces drivers to slow down” and decreases the likelihood of head-on collisions, resulting in fewer fatal crashes. The continuous flow of traffic also decreases pollution, since vehicles spend less time stopped at a red light.
“Roundabouts are an effectual device that when compared to traffic signals and all-ways control intersections have been shown to reduce all crash types, but more importantly, the severe and fatal injury crash type,” Colby GeDeros, ACHD traffic engineering supervisor, told the Idaho Statesman by phone.
According to GeDeros, roundabouts are also safer for pedestrians and bicyclists than traditional traffic-signal intersections because they are crossing the intersection with vehicle traffic coming from only one direction.
When the Eagle-Amity connection was a single-lane roundabout, accident rates were at 0.49 crashes per million entering vehicles during a five-year period (2016-2020), according to ACHD monitoring data. In comparison, there were 0.66 crashes per million entering vehicles during the same period at the Eagle and Victory Road traffic-signal intersection.
After ACHD turned Eagle-Amity into Ada County’s first and only dual-lane roundabout, accident rates for the intersection began to increase. The dual-lane roundabout has resulted in more crashes than those recorded when the intersection was still a four-way traffic signal intersection.
“That’s not necessarily unusual,” GeDeros said. “(It’s) just inherently more complex than a single-lane roundabout.”
What makes dual-lane roundabouts different?
A dual-lane roundabout has at least two lanes entering and exiting at each opening, with two full lanes going around in a circle. Its traffic rules also make it unique, meaning a larger potential for accidents for those who may not know how to use roundabouts properly.
Switching lanes and passing other cars while inside a roundabout is not allowed. Drivers are also expected to leave room for larger vehicles (such as semi trucks or school buses) that may take up more than one lane while turning.
“We do sometimes see resistance to roundabouts,” Rachel Bjornestad, public information officer for ACHD, told the Statesman by phone. “A lot of it is just the fact that they’re new, they’re different. I think people can be resistant, because they may not be sure how to use them correctly or may fear that others don’t know how to use them correctly. But also what we find is once the roundabout is in and people start knowing how to use it, they actually do like them, because it does help with the flow of traffic.”
While structuring a roundabout in this way can leave room for more crashes, GeDeros said the number of fatal crashes has significantly decreased. In other words, of the increased number of vehicle accidents since the traffic signal intersection became a dual-lane roundabout, fewer of them have resulted in serious injuries.
“We’ve seen no bicycle- or pedestrian-related crashes, and that’s what we expect,” GeDeros said. “Roundabouts have been found to be extremely safe for pedestrians and bicyclists nationwide.”
If turning traffic-signal intersections into single-lane roundabouts eliminates head-on vehicle collisions and decreases traffic accidents, why turn them into dual-lane roundabouts in the first place? Why not simply keep them as single-lane roundabouts if adding lanes will increase accident rates?
According to GeDeros, the highway department’s intent is to “size the roundabout for immediate and near-future vehicle demand,” meaning that if a single-lane roundabout can efficiently carry vehicle volumes for 10 years, ACHD will most likely refrain from expanding it.
“Our goal is to basically build these roundabouts as small and as least-complex as possible, so long as we can get roughly 10 years of capacity out of that,” GeDeros said.
Safety measures for roundabouts
According to the highway district, the increase in crashes since the intersection was turned into a dual-lane roundabout is consistent with the vehicle accident data researchers have found for other states.
“We have had an open dialogue with law enforcement on this roundabout, and they’ve informed us of, you know, responding to crashes at this intersection relatively frequently,” GeDeros said. “We’ve done some in-depth analysis of the crashes and we’ve made some improvements … to try to make it more clear exactly what people are supposed to be doing as they approach and go through the roundabout.”
Improvements include additional signs reminding drivers to yield before entering the roundabout, and pavement markings signaling which lanes drivers should be in, and what movements they’re allowed to make.
Rapid-flashing beacons, crossing devices that flash warning lights at vehicles when pedestrians are waiting to cross, were added to the Eagle and Amity roundabout. According to GeDeros, the beacons will become standard for all other dual-lane roundabouts from now on.
ACHD’s plans make clear: Roundabouts are not only here to stay, but to expand.
ACHD has proposed 12 roundabouts on Amity Road between McDermott and Maple Grove. Four are dual-lane, three are single-lane, and four are multilane roundabouts, which are single-lane roads that expand to two lanes while inside the roundabout. The 12th is a triple-lane roundabout, one of at least five planned on Amity (one), Ustick Road (three) and Victory Road (one).
Two of the proposed roundabouts, the one at Eagle and Amity and the one at Ten Mile and Amity, have already been completed.
“It’s not guaranteed that all these planned roundabouts will happen,” Bjornestad wrote in an email to the Statesman. “It’s subject to change based on things like traffic studies during the design phase, or direction from the ACHD Commission.”
