US-20B Stockham and US-20 “Peanut” Roundabout | Rigby, ID

Project Overview

Horrocks provided planning, survey, and design services for the US-20B Stockham and US-20 roundabout in Rigby, an innovative intersection improvement that became the first roundabout on ITD’s system and the first nontraditional, “odd-shaped” roundabout in the region. Successfully fast-tracked to meet an aggressive schedule set by ITD and project partners, the project significantly improved safety, access to local businesses, and corridor mobility while introducing advanced 3D modeling and Automated Machine Guidance (AMG) construction techniques to ITD.

Services Provided:

• Intersection planning and geometric design for a constrained, unconventional roundabout configuration

• Topographic, utility, and right of way surveys, including LiDAR collection for high-voltage transmission line clearances

• Advanced 3D modeling of roadway geometry, drainage, utilities, and grading

• Development of a fully integrated AMG construction model to guide contractor equipment directly from the digital design

• Vehicle tracking analysis for passenger vehicles, school buses, farm equipment, and long-haul trucks

• Field testing and visualization support to validate design assumptions

• Utility modeling and coordination using 3D data shared through Google Earth

• Public outreach support through visualizations, videos, and educational materials

Challenges

The existing “T” intersection geometry created safety concerns, operational inefficiencies, and difficult turning movements, particularly for large trucks accessing adjacent businesses. Rapid area growth and mixed vehicle types, including freight trucks, farm equipment, school buses, and passenger vehicles, further complicated the design.

As ITD’s first roundabout, the project required absolute confidence in performance and clear communication of design intent to reviewers, stakeholders, the contractor, and the public. The roundabout’s unconventional “peanut” shape generated early concerns about the ability of large trucks to navigate the intersection safely.

Horrocks addressed these challenges through extensive vehicle tracking analysis and a full-scale, real-world field test. A temporary mockup of the roundabout was painted in a local trucking yard, where typical vehicles and a WB67 truck were driven through the layout to confirm operational performance. Aerial video from the test became a powerful public outreach tool, helping build understanding and trust in the design.

Successes

• Successfully delivered ITD’s first roundabout and the region’s first nontraditional roundabout

• Fast-tracked the project to meet an aggressive ITD schedule

• Improved safety and operational performance for all vehicle types

• Enhanced access to adjacent businesses while minimizing delay for through traffic

• Implemented innovative 3D modeling and AMG construction delivery

• Eliminated traditional construction staking, improving efficiency and reducing rework

• Built public and stakeholder confidence through visualization, field testing, and education

Through advanced modeling, collaborative problem-solving, and proactive public engagement, the US-20B Stockham “Peanut” Roundabout set a new precedent for intersection design in Idaho. The project not only resolved longstanding operational challenges but also expanded ITD’s toolkit for innovative, constructible, and community-supported transportation solutions statewide.