Project Engineering Consultants, Ltd. (PEC) was responsible for all aspects of the project from concept design to construction engineering and inspection. PEC’s work included a concept study to determine the preferred improvement alternative, the preparation of full construction plans and specifications for an expandable single-lane roundabout, a robust public involvement program and construction engineering and inspection.
When the concept study for this project began, typical practice throughout the Boise-Nampa metropolitan area for improving this type of intersection was to add turn lanes and traffic signals. While this approach would have provided at least temporary relief from the traffic congestion, it would not have addressed the safety issue to the level that the introduction of a roundabout would. PEC recognized that the traffic and topographic characteristics of the intersection appeared, in all respects, to be amenable to roundabout implementation. For this reason, PEC recommended that the City consider a roundabout as one of the alternatives in the concept study. The City agreed, and the roundabout eventually won out as the preferred alternative.
The introduction of an arterial roundabout to Southwest Idaho was, in and of itself, a pioneering endeavor. There are also some innovative aspects to this specific project relative to other roundabouts. These included creative methods of both roundabout design and public outreach.
An innovative aspect of the design is the roundabout’s expandability. One issue widely acknowledged by roundabout experts is the undesirability of building roundabouts with more lanes than will be required in the near future due to the operational difficulties and loss of safety benefits that can result. In recognition of this issue, it is common for single-lane roundabouts to be constructed that are later torn out and replaced by multi-lane roundabouts. PEC addressed this issue by first developing a dual-lane design to meet the intersection’s long-range capacity needs and then phasing the construction to initially build it as single-lane. This approach saved the City money both at the time of initial construction and at the time of later expansion as compared to more traditional approaches.
Another PEC innovation was the public outreach program on this project which was greatly enhanced through a roundabout “trial.” A full-scale layout of the proposed roundabout was set up in a large parking lot at the Idaho Center in Nampa. Public safety agencies, bus companies, truckers, contractors and the general public were all invited to drive through the roundabout mock-up to experience for themselves how the roundabout would work and how easily their large vehicles could be accommodated. The local television and print media gave the event extensive coverage. This event, combined with the traditional open house-style public outreach, was instrumental in gaining public acceptance for the project.
PEC’s approach to the expandable roundabout design has the potential to help roundabout designers across the country to design roundabouts that are truly expandable, rather than design single-lane roundabouts that must later be replaced by dual-lane ones. Not only will this help to reduce costs to project owners, but it will also help make roundabout construction a viable alternative at locations where future expansion needs would otherwise render it infeasible.