Why does the queue explode as arrivals approach the service rate?

Because arrivals are not evenly spaced. Even when the average demand sits below capacity, a run of vehicles arriving close together builds a queue that the gaps afterwards have to clear, and as the two rates converge the gaps stop being long enough. Past about eighty percent utilisation the design queue lengthens far faster than the demand does, which is why a small increase in traffic can double a throat requirement.

What if demand is higher than the control point can serve?

Then no throat length works, and the page says so. A queue with nothing to clear it grows for the whole peak, so the answer is more capacity rather than more pavement: a second lane at the gate, a faster transaction, transponder access for regulars, or another way into the site. Building a longer throat only moves where the queue ends up.

Is the throat measured from the street or from the property line?

From where the vehicle stops being in the street. The distance that matters runs from the through lane, across the apron and the sidewalk, to the first point where a queued vehicle would block a parking aisle — and the traffic authority will usually measure it from the right of way line. Draw it on the plan rather than quoting a single number, because the two ends have different owners.

Does the first aisle really matter as much as the street?

It matters differently. Spillback into the street is a safety problem the highway authority will refuse the plan over; a queue standing across the first aisle traps everyone already parked and produces the reversing manoeuvre into a moving queue that fills a site's incident log. Both are measured from the same throat, so both are solved or neither is.

Driveway Throat Queue Length Calculator

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Peak Arrival Rate (vehicles per hour)

Vehicles entering during the busiest hour of a design day.

Use the peak hour, not a daily average divided by the hours it spans — a school, a stadium and a shift-change industrial site all deliver most of a day's arrivals within a few minutes, and averaging hides exactly the condition the throat exists for. A trip generation reference gives the volume where counts are not available.

Service Rate at the Control Point (vehicles per hour)

How many vehicles the gate, kiosk or window can process each hour.

Measure it if the facility exists, and be pessimistic if it does not. A gate arm that must rise, clear a vehicle and fall again, or an attendant taking payment, serves far fewer vehicles per hour than the smooth figure a supplier quotes, and the throat is sized on the real rate.

Design Percentile

How often the design queue is allowed to be exceeded.

A queue has no maximum, only a distribution, so a throat is designed to a percentile rather than to a worst case. The 95th percentile is the ordinary choice; move up where the consequence of spillback is a vehicle stopped in a high-speed through lane, and check what the access management standard for your street class requires.

Storage Length per Queued Vehicle

Vehicle plus the gap a stopped driver leaves in front of them.

This is a queue storage figure rather than a vehicle length: drivers leave a gap when stationary, and the gap is part of the space the queue occupies. Raise it where the traffic includes a meaningful share of trucks or vehicles towing.

Allowance at the Head of the Queue

Extra length for the longest vehicle expected to be served at the control point.

The vehicle at the gate is the one that has to clear the arm before it drops, and a long one occupies far more than a queue slot while doing so. Set this to the difference between the design vehicle and an ordinary car, and remember the reader or call box must sit where a high cab can reach it.

Throat length required

162 ft

Medium confidence

A steady-state single-server model. Real arrivals cluster, service times vary, and a queue that reaches the street is a different problem from one that reaches the first aisle — check both ends against the plan.

Design queue: 6 vehicles
Demand against capacity: 0.6 ratio
Storage applied per queued vehicle: 25 ft

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Single-server queueing model: with demand-to-capacity ratio p, the chance that n or more vehicles are present is p to the power n, so the queue not exceeded in a chosen fraction of the peak hour is ln(1 − fraction) ÷ ln(p). Throat length is that queue times the storage length per vehicle, plus an allowance at the head for the longest vehicle expected.
ITE Trip Generation Manual and ITE Parking Generation Manual — where the peak arrival volume this model is driven with comes from. Neither publishes a throat length.
Local access management standards set the minimum throat length and the design percentile for a given access class. This page is the arithmetic behind that requirement, not a substitute for the adopted figure.

Inputs used

Peak Arrival Rate (vehicles per hour): 180
Service Rate at the Control Point (vehicles per hour): 300
Design Percentile: 95th percentile — the usual design basis
Storage Length per Queued Vehicle: 25 ft
Allowance at the Head of the Queue: 11.5 ft

Intermediate steps

Design queue: 6 vehicles
Demand against capacity: 0.6 ratio
Storage applied per queued vehicle: 25 ft

Final result161.5 ft

Confidence note: A steady-state single-server model. Real arrivals cluster, service times vary, and a queue that reaches the street is a different problem from one that reaches the first aisle — check both ends against the plan.

What this calculation does not cover

Two failure directions share one dimension: a queue that spills into the street, and a queue that blocks the first aisle so parked drivers cannot leave. Both are measured from the same throat.
Nothing here checks the curb return radius, the apron cross slope, or the pedestrian route that crosses the drive.

Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0

Regulatory standards & verification citations3

Single-server queueing model: with demand-to-capacity ratio p, the chance that n or more vehicles are present is p to the power n, so the queue not exceeded in a chosen fraction of the peak hour is ln(1 − fraction) ÷ ln(p). Throat length is that queue times the storage length per vehicle, plus an allowance at the head for the longest vehicle expected.
ITE Trip Generation Manual and ITE Parking Generation Manual — where the peak arrival volume this model is driven with comes from. Neither publishes a throat length.
Local access management standards set the minimum throat length and the design percentile for a given access class. This page is the arithmetic behind that requirement, not a substitute for the adopted figure.

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Driveway Throat Queue Length Calculator

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