Car wash site and stacking calculator

How this works

A stacking lane holds the cars waiting to enter the wash, and in nearly every code our research reviewed it is set as a fixed count of spaces. Lynnwood requires 2 per wash bay plus the vehicle in the bay, with 1 space beyond the bay, or 3 if hand drying or detailing, under LMC 21.18.810 ^[1]. Pompano Beach requires 4 per bay for an automatic car wash under Table 155.5101.G.8.a ^[2], and Maricopa requires 4 per bay for an automated or self-service wash and a minimum of 8 for a full-service wash under MCC 18.120.090 ^[3]. A preliminary site plan carries that count in its site data, against the stacking the layout provides.

Choose the jurisdiction and the category its code uses, and the calculator reads that one row. Texas City requires 3 per self-service drive-through or automated bay, plus the bay, and 7 for a full-service wash, a vacuum lane or a pump lane under §160.081(C) ^[4]. Cedar Hill requires 3 per automated bay plus the bay, and 4 per lane for a full-service wash ^[5]. Houston counts 2.5 spaces per bay or stall for stacking at an automated wash and 1 per bay at any other car wash under Sec. 26-492, Class 9 ^[6]. Philadelphia's Table 14-805-1 sets 6 stacking spaces for a full-service car wash ^[7]. Where a row says per bay or per lane, the calculator multiplies it by the bays or lanes entered; where it states a count, the calculator shows that count and does not multiply it. The car wash site plan page shows how the stacking lane shapes the rest of the layout.

Which category a conveyor tunnel or an in-bay automatic falls in is set by each code's own definitions, which our research did not verify, so the calculator asks for the category in the code's words and never assigns one.

The queue estimate

Fixed counts are the usual rule, but where a code or a reviewer asks for an analysis, the measure is the 95th-percentile queue. An approved transportation impact study for a car wash in Pittsburg, California states: "The 95th-percentile queue is generally applied as the acceptable limit for on-site circulation impacts" ^[8]. TxDOT's manual defines the 95th-percentile queue as the queue with "a five percent probability of being exceeded" ^[9].

The calculator models each tunnel, bay or lane as a single-server queue (M/M/1) and splits the design arrivals evenly among the wash lines. Utilization is the arrival rate at one line divided by its service rate. In that queue, the chance of more than n vehicles at the line is the utilization raised to the power n plus one, so the 95th-percentile queue is the smallest whole n for which that chance is five percent or less, the definition TxDOT's manual gives ^[9]. When arrivals reach or pass the service rate, utilization is one or more and the queue has no steady state: it grows without bound at that rate, and the calculator says so instead of giving a figure. The count includes the vehicle being served, so where a code counts its stacking spaces plus the bay, the estimate holds one vehicle the code counts separately.

The service rate matters more than any other input. In the Pittsburg study, the operator put the tunnel at about 60 vehicles per hour, 1 vehicle per minute, with no tunnel length stated ^[8], and the calculator uses that operating rate, labelled observed in approvals, unless you enter your own. An equipment design calculation sheet puts nominal capacity at the tunnel length in feet per hour, 100 per hour for a 100 ft tunnel, at a standard conveyor speed of tunnel length divided by 150 seconds, with a 17 ft vehicle ^[10]. The calculator shows that manufacturer figure beside the estimate and never sizes the queue to it. No row gives a service rate for an in-bay automatic or a self-serve bay, so for those the queue needs the rate you expect the bay to serve.

One study in our research gives queues observed at working washes, and the calculator shows it beside a tunnel estimate, labelled observed in approvals. A traffic evaluation dated 11/26/2025, filed for a car wash in West Town, Pennsylvania, reports an average peak queue of 16 vehicles on a Saturday midday, observed at 4 sites in Maryland and Virginia with 5-minute observations, not at the West Town site ^[11]. The express car wash in Mansfield, Texas and the express car wash in Hillsborough County, Florida case studies show stacking lanes drawn for that kind of demand.

Space size and lane length

Codes size each stacking space as well as counting them. Lynnwood's space is 20 ft x 10 ft, 12 ft wide on curves ^[1]; Texas City's is 8 ft x 20 ft ^[4], and Cedar Hill's the same ^[5]; Pompano Beach's is 9 ft x 18 ft ^[2]; and Maricopa's is a 12 ft x 20 ft aisle ^[3]. Philadelphia sets a 10 ft minimum lane width, a 20 ft setback of the stacking lane from property lines, and a raised median, curb or landscape between stacking and bypass lanes ^[7]. Where the jurisdiction's space size is verified, the calculator multiplies the 95th-percentile queue by the space length to give the stacking lane in feet per wash line. The Texas car wash site plan and Florida car wash site plan pages cover the state approval paths around that lane.

Trips, traffic studies and turn lanes

ITE's 11th Edition counts an automated car wash, land use 948, by its tunnels, and a traffic memo filed in California states that "ITE does not have an ADT rate or AM Peak Hour trip rate for Land Use Code 948" ^[12]. A car wash trip generation memo filed with Lee's Summit, Missouri, reports 78 PM peak trips for one tunnel and notes that AM and daily are not provided ^[13]. The calculator multiplies that rate by the tunnels entered and labels it with its edition. A self-serve wash is counted per wash stall under land use 947, at 5.54 PM peak trips per stall, a value the 10th and 9th Editions share ^[14]; no row gives a rate per in-bay automatic, so none is computed for one.

Traffic study thresholds are set by each state's transportation department for access to its highways, and the calculator shows the rows of the jurisdiction's state. Arizona's TGP 240 calls for a traffic impact analysis at 100 or more trips in any one hour, Category I from 100 to under 500 and Category II at 500 or more, and a traffic statement under 100, including turn-lane analysis per TGP 245/430 ^[15]. Pennsylvania's policy of 6/4/2025 sets 3,000 or more trips per day, or 150 or more entering or exiting in any one hour ^[16]. Minnesota recommends a study at 250 or more peak-hour trips or 2,500 new daily trips ^[17]. Texas has no single statewide trip threshold, and TxDOT's Access Management Manual says its scenarios "should not be used as thresholds" ^[18]; the TxDOT Houston District calls for a major study at 150 or more peak-hour trips or 1,500 or more daily trips ^[19].

The calculator compares its PM peak estimate only with a trigger that counts trips in a peak hour or in any one hour, and states every other trigger as written. At 78 PM peak trips, one tunnel stays below Arizona's 100-trip line ^{[13, 15]}. Turn-lane warrants count turning vehicles at a driveway, which the calculator does not estimate. Arizona's TGP 245 sets right-turn lanes by advancing volume: for 1 lane at 45 mph or more, 30 vph at 201 to 300 advancing, falling to 3 at over 1,400 ^[20]. Left-turn lanes are warranted at 30 vph under 45 mph, or 15 at 45 mph or more, when advancing volume is under 200 ^[20]. The calculator shows those rows as stated.

Where a city's parking table names a car wash, the calculator adds it by the code's own condition: Houston requires 2.5 per bay or stall for stacking at an automated wash and 1.0 per bay or stall for all other car washes ^[6]. The parking requirements lookup holds the same rows beside other uses, and the car wash visualization page shows how a stacking lane reads in a rendered site plan.