Site Access Design

How we approach it

Designed to the standard, approved the first time.

Site access is where private design meets public infrastructure, and where the most expensive surprises come from. Street design follows guidance from AASHTO, ITE, and FHWA, with local modifications for regional conditions — and in the Treasure Valley those local modifications come from ACHD in Ada County and from the relevant highway district or county in Canyon County. Each agency has its own access policy, its own spacing standards, and its own thresholds for triggering a traffic impact study. We work with all of them on every project.

The first design question is always sight distance. The driver leaving your site has to see far enough up the road to merge safely; the driver approaching your site has to see the entrance in time to slow down, signal, and turn in. AASHTO publishes the standards (intersection sight distance and stopping sight distance, both keyed to design speed), and they are non-negotiable. If the parcel doesn't have the sight distance, the access doesn't work — and Bailey identifies that constraint at feasibility, not at construction documents.

The next question is queuing. A driveway throat that's too short forces drivers to queue out into the public road, which kills capacity and creates rear-end collision risk. The core question is straightforward: is the driveway length adequate to allow for vehicle queuing? On most retail and commercial sites the answer requires a real calculation, not a guess. Drive-throughs make it harder. Multi-tenant centers make it harder. Bailey runs queuing analysis against expected peak-hour demand and sizes the throat for the worst-hour reality, not the average-hour assumption.

Turn-lane geometry is the next layer. A right-turn deceleration lane keeps right-turning traffic from blocking the through lane behind it; a left-turn lane keeps left-turning traffic from being rear-ended in the median. Each has its own warrant: based on volumes, design speed, and the agency's own policy threshold. Where the warrants are met, we design the lane to ACHD or AASHTO geometry. Where they're not, we document why and ask the agency to confirm.

Right-in/right-out access has its own design problem. On multi-lane arterials with raised medians, full-movement access is often impossible without a signal — and signals have warrants too. The design alternative is right-in/right-out with median treatment, sometimes paired with a downstream U-turn. We model the trip patterns against the access configuration and tell clients early whether their pro forma assumes a movement that the agency won't approve. Catching that mismatch in feasibility saves real money.

Finally, the on-site half: once a vehicle is on the parcel, the internal circulation has to deliver it to parking, loading, drop-off, or service without conflict. Loop drives, perimeter circulation, parking aisle orientation, and landscape island placement all flow from the access points inward. We design the access and the internal circulation as one system, because they have to work as one system from the day the site opens.