In an innovative measure to create a safer intersection of two high-speed rural routes and reduce the severity of collisions, Caltrans is constructing a turbo roundabout at the intersection of Highways 25 and 156 in San Benito County, adjacent to Hollister Municipal Airport. Graniterock is the general contractor on the project.


Traffic is led into the new Turbo Roundabout, which will receive next, in its final phase, striping for the three lanes, along with overhead and ground-level directional signage.

Serving both commuters and trucks transporting key goods, the old standard-design intersection had been experiencing more than twice the number of broadside and rear-end collisions than similar intersections in California. Adjusted signal timing, “signal ahead” signs, flashing beacons, and enlarged signal heads all provided some benefit, but not enough to significantly reduce the high number of accidents at this intersection.


The embedded rock median leads up to the Turbo Roundabout from all four directions. When each directional approach nears the roundabout, lane dividers (D-Mod curbs) separate the three lanes going into the roundabout, as depicted in the drawing below.

turbo roundabout

A turbo roundabout operates in a similar fashion to a standard roundabout, but has additional safety features that reduce the potential for collisions. Three-inch lane dividers, overhead signage, and pavement marking are three of the key safety additions.

At the interior of the turbo roundabout, a truck apron has been constructed to accommodate the sweep of a semi trailer’s wheels as it navigates through the interior of the roundabout.

Construction of the $10.9-million turbo roundabout is slated to complete this month. Final striping, and overhead and ground-level sign installation to wrap up the project is now underway.

To view the Caltrans and Council of San Benito County Governments demonstration video, click this link:

Caltrans Turbo Roundabout Explained 

A tip of the hard hat to Graniterock Project Manager Mark Hurley, Jr. for the photos, drawings, and project information.