Reservoir Space: Sizing the Parking Garage Entry Queue

Reservoir space is the length of drive lane between the street and a parking facility’s entry control point where arriving vehicles can wait without blocking the sidewalk or the travel lanes.

At a glance

Also called: Stacking space, queuing reservoir
Sized by: A queuing analysis of peak arrivals and the time each car spends at the entry
Key fact: When the line outgrows the reservoir, it spills across the sidewalk and into traffic

The reservoir is the stretch of drive lane between the sidewalk and the gate. If the line outgrows it, the next car waits across the sidewalk or in the travel lane. Illustrative.

Every parking entry is a small service line. Cars arrive in bunches, especially before an event, a shift change or the start of a school day, and each one stops for a moment at the gate while a ticket prints, a card reads or a plate is recognized. The reservoir is the space where that line forms. Planners usually express it in vehicles and allow roughly 20 to 25 feet of lane for each waiting car.

What sets the size

Peak arrival rate. The busiest quarter hour of the year, not the daily average.
Time at the entry. Paying at the gate is slowest, a ticket or credential is quicker, and a license plate read with no stop is quickest.
Number of entry lanes. A second lane roughly doubles what the entry can process.
Bad days. A jammed ticket, a failed reader or a confused first-time visitor stops the line for everyone behind.

Queues do not grow in a straight line. As arrivals approach what the entry can handle, random bunching makes the longest line grow several times over, which is why a reservoir that looks generous on an average day can fail at the peak.

Try it

Will the line reach the street?

Set the peak arrivals, how drivers enter, the number of lanes and the length of the reservoir, then play the peak.

With one lane and pay-at-entry (about 20 seconds a car), 60 arrivals in 15 minutes exceed what the gate can process and the line runs into the street. Switching to a ticket or card (about 8 seconds) keeps it inside a six-car reservoir.

Arrival pattern, processing times and the 22 feet per car are illustrative.

Where it goes wrong

Gates placed at the property line, so the first waiting car sits on the sidewalk.
An entry that starts climbing a ramp immediately, with no level stretch to queue on.
Payment at the entry in a facility with sharp arrival peaks.
Entry and exit sharing a short driveway, so outbound cars wait behind the inbound line.

The Parkonomics view

Reservoir space costs almost nothing on paper and is nearly impossible to add later, because the gate, the ramp and the property line are fixed in concrete. Boise’s parking structure design guidelines call for a queuing analysis prepared by a parking professional and require gates to sit far enough back to hold at least one vehicle off the sidewalk. Treat the reservoir as part of the street design, not the garage design, because that is where its failures land.

Further reading: Designed to Park, or Designed to Impress?