As a site planner, I usually provide 25-foot vertical drive aisles with 30-foot cross aisles if two-way traffic is allowed. I've done layouts with 15-20-foot one-way aisles.

Turning radius is defined as the diameter of the circle created by the outer front wheel when making a full turn - or u-turn. There are two ways of measuring the turning radius: curb to curb and wall to wall. The latter is always larger because it takes into account front-end overhang. The numbers published in new car brochures are usually the curb to curb numbers.

I have seen numbers like 33-35 feet for a compact, 35-37 feet for a mid-size, and 36-40 feet for an SUV. Divide these numbers in half for a 90-degree turn.

You may have to be more concerned about the turning radius of a fire truck to get your layout approved. Figure roughly 41 feet curb to curb & 44 feet wall to wall for a typical fire truck (non-ladder truck).