The stall gets the attention, but the module does the work. A module is the full bumper-wall-to-bumper-wall dimension: the drive aisle plus the stall depth on one side (a single-loaded module) or both sides (a double-loaded module). It is the unit in which parking geometry actually trades, because a driver's turning comfort is a joint product of stall width and aisle width: within reasonable limits, the same comfort comes from a wider aisle with a narrower stall or a wider stall with a narrower aisle. Design the module, and the stalls and aisle fall out of it.
The science under the dimensions. Module sizing stopped being trial and error in the 1950s, when Edmund Ricker modeled the turning movement of a vehicle into a stall as a set of equations, refined since and still in service. Combined with accumulated field experience, the equations yield the recommended minimum module dimensions by parking angle that the reference tables carry. The tables' quiet sophistication is what rotates: the geometry is based on rotating the design vehicle to the parking angle, not rotating the painted stall. Because the design vehicle is smaller than the stall, the space between the front of the parked vehicle and the head of the stall is real, usable aisle width that the painted-lines method would waste. Basing the module on vehicles rather than stripes buys a more efficient layout, slightly narrower aisles, and no loss of driver comfort, which is the whole game of rational design in one move.
The trades the module permits. The published dimensions are minimums, and the adjustments run in known directions. Where comfort must rise, widen the stall and shrink the module: patrons feel every added inch of stall width and barely register a tighter module, and the working exchange rate is one inch of added stall width for three inches of module reduction. Where a physical restraint (curb, wall, bumper stop) fixes how far vehicles can pull in, the aisle can safely give up a foot, because the restraint eliminates the creep that otherwise steals aisle width; where no restraint exists, as in the open middle of a shopping-center lot, vehicles pull through and overhang, and the Snowbelt makes it worse when snow and salt erase the markings entirely. The unrestrained module keeps its full dimension for a reason.
Interlock, the herringbone dividend. With angled parking, stalls in adjacent modules can align nose-to-nose in a herringbone, each overlapping into the other's module; that overlap is the interlock dimension, and it is the one table value that varies with stall width. Interlock is free efficiency when the layout earns it, and it is also the first thing encroachments revoke: the standing rule, detailed with the column rules in the next module, is that interlock reductions must not be taken where obstructions intrude into more than 30 percent of the stalls.
design in modules, not stalls: start from the recommended minimum module for the angle, base the geometry on the rotated design vehicle rather than the painted stall, and buy comfort with the one-inch-stall-for-three-inch-module trade. Take the one-foot aisle reduction only where a physical restraint enforces stall position, harvest interlock where the herringbone aligns, and treat every table value as a minimum that user needs can raise but nothing should quietly shave.
From the shelf
- Module 16: the 61-ft building blockthis module at building scale
- Module 23: stall dimensionsthe dimension set, shared
Source crosswalk -- where each section came from in the manuscript
| Module section | Source: Chapter 9, "Parking Geometrics" |
|---|---|
| Module definition | "Determining The Dimensions Of Drive Aisles And Modules" (aisle, single/double-loaded) |
| Ricker and rotation | Same (1950s equations; design-vehicle rotation vs. painted space) |
| Trades | Same (minimums; 1"-for-3" trade; 1-foot restraint reduction; Snowbelt pull-through) |
| Interlock | Same (definition, width-varying value, 30% linkage) |
| Not carried forward | Stall widths (in #23); encroachment rules (in #25); guidelines/user framing (in #23) |