STANDARD GUIDELINE FOR AUTOMATED PARKING FACILITIES in the UNITED STATES

This Standard-Guideline [SGL] will describe the design parameters, construction and operation of Mechanical Parking Systems [MPS] and Automated Parking Facilities [APF] in the US. MPS and APF may consist of simple devices that stack two or three vehicles on a single parking space and range to sophisticated fully automated facilities in which multiple robots perform the storage and retrieval of vehicles through computerized processes without human intervention. Sizes range from 2 spaces to several thousand parking spaces.

It is not the intention of this SGL to specify detailed structural, mechanical, electrical and electronic issues as these are well covered in reference codes mentioned below. Moreover this SGL intends to provide an overview of key features and implementations typically required by building codes as well as familiarize Authorities, owners, designers and the public with this type of parking system as to layout, typical dimensions, fire and life safety features, accessible design, vibration and sound attenuation, traffic and access design considerations. This SGL also addresses operational aspects for the operation of Automated Parking Facilities and their effect on public streets traffic, queuing and throughput capabilities.

Table of Contents

1 Nomenclature
2 Mechanical and Automated Parking
3 Use of the garages, size and infrastructure needed
4 Requirements on slot sizes
5 Automated Parking Facilities
    5.1 Location, Size and components of Terminals
    5.2 Function of Terminals and Public Areas
    5.3 Throughput Capability
    5.4 Traffic Effects and Queuing
    5.5 ADA requirements
    5.6 Fire Safety
    5.7 Sound Emissions - Vibrations
    5.8 Zoning / Codes for M/A systems
    5.9 Graphical User Interface
    5.10 Ventilation / Heating / Air Conditioning
    5.11 Lighting/ Accessibility for Maintenance
    5.12 Diagnostics
    5.13 Maintenance / Operator
    5.14 Spare Parts Package

1   NOMENCLATURE (Definition of the Terminology used in this SGL)

Automated Parking: All the steps necessary to initiate, perform and control the process of vehicle storage or retrieval utilizing a multiple of machines with vertical and horizontal transport capability operated via software commands to the different machines. No human intervention needed.

APF : Automated Parking Facility

AVI : Automated Vehicle Identification; A transponder mounted at the vehicle entry automatically identifies and accepts vehicles with properly issued identification tag access to the facility.

Carriers : A certain type of apparatus in a M/A system performing horizontal movements in x directions

Controls : The part of a system which is comprised of computers, sensors, PLC’s; in short the entire electronics

Credit Card Access: A patron accesses the parking facility by inserting a credit card into the entry reader, which provides for automatic payment of the parking fee allow patrons to use the M/A system via the use of a regular credit card

DTO : Dynamic Traffic Operation

DVO : Dynamic Valet Operation

DwellTime : The time it takes a Patron to drive into the Terminal and walk out to the point of system activation

HMI : Human Machine Interface, another word newer date for GUI or MMI ( Man Machine Interface)

Lifts : Transport devices used for vertical movement of vehicles (not to confuse with elevators)

Lobby : The designated area in an M/A system where patrons go to request their car for retrieval

Machinery : Another word for lifts, shuttles and carriers is not one single apparatus as there are specific words

M / A : Mechanical / automated

Mechanical System: Method of parking cars that involves a person to some degree (garage attendant or driver of car) to take necessary steps to initiate/perform mechanical movements, using different machines, which are typically triggered by electrical pushbuttons, not a software program.

Pallets : A platform on which the car is transported and stored.

Patrons : Persons driving a car and using the M/A System for parking their vehicle

PLC : Programmable Logic Controller; an industrial computer performing tasks written on the so called inherent Ladderlogic (a software program written like a ladder, line by line in sequence following logical commands)

POF : Pay On Foot; A device which allows automatic cash or credit card payments of the parking fee and interfaces with the M/A parking system to store and retrieve vehicles.

Rack : The supporting frame for a M/A parking system, providing the necessary framework for storage, machinery and eventually the building around the M/A parking system.

Rail : fixed runway for a single or multiple machines in a M/A system

Retrieval time: The time measured from the moment of the activation of the retrieval process by the patron until the patron can access its car to drive out of the M/A system

RF : Radio Frequency; a means of communicating signals without wires but airwaves on certain frequencies

RF Tag : A card similar to the size of a credit card which transmits a defined signal to a reader which in turn communicates this signal to the main computer of the M/A system

Shuttle : A vehicle transport device that moves horizontally in x and y direction.

Slot : A storage space for one car inside a M/A system

Spaces : The amount of parking slots a M/A system can hold / park for a certain configuration of the system

Storage time : The time measured from the moment a patron activates the system to store its car after proper positioning of the car in the Terminal until a next patron can use one free terminal to enter with its car for another store

System : The entirety of the M/A facility including structure, mechanics, electric, electronic and software with hardware

Terminal : The drop-off and pick-up point of a M/A system

Throughput : The amount of cars in any one way flow, which the system can perform in a specified time frame

Ticketing Station: A form of access to the M/A system which dispenses a ticket to the patron. Located at the left outside of the outer door of the Terminal

2   Mechanical and Automated Parking

2.1   Mechanical Parking

By definition, mechanical devices perform a predetermined specific physical work without human intervention. By the push of a button or switch a single step movement is initiated to store or retrieve a vehicle. Examples: double or triple stacker, carousels, any set-up of fixed mechanical movements initiated directly by humans.

Typically mechanical systems serve for the storage of 2 to 30 cars per unit.

 

2.2   Automated Parking

Automated Garages are also based on mechanical devices, but they differ in so far that a) multiple mechanical devices are present and can operate simultaneously to store and retrieve vehicles in a multiple rack structure vertically and horizontally, and b) the operation is not directly initiated by human intervention, but by a software program specifically written for this system that controls the movement of all transport devices. Its management software tracks the location of the vehicles stored in the facility and returns the vehicles, requested from the patron upon initiation of the access system installed. Automated Parking Systems are designed to receive one input from the patrons for the storage or retrieval of the vehicle. It initiates and controls all necessary commands and steps in proper sequence by software commanding the devices to store or retrieve the car. This system is most often characterized as automated valet parking.

Automated parking facilities are geared to store and retrieve in a fully automated way 50 to several thousands of cars.

3   Use of the garages, sizes, infrastructure

Mechanical Systems are best suited for capacities of 2 to up to 40 cars per unit, and are suited for any type of land use.

Automated Parking facilities are best suited for larger developments consisting of residential, office, hotel, entertainment, shopping, airport, hospital and university campus projects, preferably with parking requirements ranging from 50 cars to several thousand. Automated Parking Facilities are particularly suited to small sites where typical ramp-access, self –park garages physically cannot work.*

Automated garages can be designed to achieve a throughput of several hundred cars per hour. Depending on the configuration of the devices and the Terminals the throughput will be determined. The manufacturer of the system determines the throughput per Terminal. the number er Terminal and devices inside the facility can then be sized to accommodate the rate of flow of trafic entering and exiting the facility.

Both these systems provide the possibility to park a multiple of cars on the footprint of one car slot using underground or above ground height. Entrances to the Facilities can be generated at random heigts.

*A ramp-access garage has a typical stall width of 8’6" to 9’0" and a typical floor height of 10 feet. The typical ramp slope used for vertical circulation and parking is 5 to 6%. Therefore, a minimum ramp length of approximately 180 to 200 feet is required for vertical circulation in a multi-story facility. This length may be divided into two parking modules that are each 60-feet wide. Therefore, the minimum size lot for a ramp-access garage is approximately 120 feet by 180 feet. This size lot is often not available in dense urban areas.

Even on larger sites, the smaller stalls and lower rack height that are possible with mechanical garages result in double the amount of parking spaces within the same volume as a typical ramp-access garage. Because mechanical parking stalls do not require additional clearance for door opening, pedestrian headroom, and turning maneuverability into a stall, they may be sized tightly to the size of the vehicle (i.e. approximately 7 feet wide by 5 feet high). The resulting lower rack heights facilitate development of parking on sites where there are height limitations above grade or a high water table or other subsurface problems in below-grade applications that prevent deeper construction. The narrower stalls also result in more spaces on any level in the garage.

4   Requirements on Slot Sizes

The minimum size / capacity of all the storage spaces and the machinery in a M/A System for standard passenger automobiles shall be 7 feet wide by 19 feet long with a minimum clear height of 5 feet. The dimensions required to accomodate vans, light trucks and sports utility vehicles shall be higher (approximate clear height of 7 feet), but not wider nor longer.

The machinery shall be capable of handling a minimum weight of 4,500 lbs per vehicle for standard passenger automobiles.

Appropriate measures must be provided to deny access to cars exceeding the capacity of the system. Also, portions of the parking facility can be designed to accommodate over-height vehicles while the majority of the spaces are dimensioned for standard vehicles or compact vhicles.

ADA requirements shall be incorporated in all M/A Systems at the Terminal or Terminals.

5   AUTOMATED PARKING FACILITIES

5.1   TERMINALS: Location, size and components

The Terminals are the most important interface zone between patrons and the Automated System. Therefore this area requires special attention.

Since cars drive into the Terminal(s) and out of it each time they use the garage, the preferred location of the Terminal(s) and of the driveways serving it is near the center of the structure on the perimeter. The preferred elevation of the Terminal(s) is that of the incoming road system.

The Terminal must be sized to allow patrons to safely and comfortably drive in and out and load / unload the cars. Turning radii and width of drive aisles shall be designed according to the respective needs with a minimum inside turning radius of 16’ and an outside radius of 28 feet. The minimum clear width of the Terminal shall be 15 feet, thus leaving 3 ½ feet left and right of the car for passengers to leave / enter the car. The Terminal access opening shall have a clear width of 15 feet for a vehicle turning directly into the Terminal. If a straight approach to the Terminal is provided for at least 30 feet in advance of the Terminal, then the width of the access opening may be reduced to 12 feet.

The length of the Terminal shall be more than19 feet.

The unobstructed height inside the Terminal shall be equals to or greater than 7 feet.

The Terminal shall be equipped with sensors to ensure the right positioning of the vehicle to be transported as well as determine the presence of oversized vehicles, protruding mirrors or racks, which exceed the size limitations of the system. Motion detectors and CCTV cameras or similar devices shall be installed inside the Terminal to ensure that no person or animals are inside the Terminal or the car when the machine starts moving.

The Terminal entrance doors shall be mounted, secured and operated safely, isolating the passengers from the Terminal during movement of the machinery and vehicles. Safety locks / emergency switches shall be installed to stop any machinery if a person or animal is detected in this area.

Cameras shall be installed to record digital photos of the physical condition of the car entering and exiting the premises. The images are also helpful to locate cars for patrons with a lost ticket and to validate damage claims by patrons.

Recesses in the floor area shall be minimized to the need of guiding the patrons in the "drive-in" process. All other areas shall be flat for pedestrian traffic. Gaps between moving parts and platforms need to be limited to ½ inch width. (compare with subways)

All M/A Systems shall comply with ADA requirements.

 

5.2   Function of Terminal and Public Areas

The Terminal is the exchange point between patron and machine and therefore special attention shall be paid to the smooth functioning of this exchange center.

The driveways for inbound and outbound traffic shall be designed to provide sufficient queuing spaces (see more on section 8); simple visual signage and guidance shall clearly direct approaching traffic off the street and into the terminals. Respective commands via a visual message center shall be applied inside the Terminals for the patrons in such manner that an easy use of the system is possible.

Inbound / outbound traffic crossing shall be prevented.

Inside and outside Terminal doors shall be provided to prevent patrons and animals from coming into contact with any moving elements of the system. The doors shall be of closed material and withstand high usage with a daily duty cycle of at least one full turnover of the garage for 50 % of the amount of Terminals.

As Terminals are the exchange station of the garage, special attention shall be directed to ease the "drive-in" and positioning of the car by the patrons (preferably by means of physical aids);

Means for catching of debris and drippings from the incoming cars shall be applied to avoid such drippings to cars and machinery inside the terminals, during transportation and storage inside the system.

The Ticketing Station or access system shall be located outside the Terminal on the left side of the inbound traffic.

If the system has installed a radio frequency access system, the readers shall have enough range to detect approaching vehicles from at least 30 feet outside of the Terminal.

The Automated Facility shall be equipped with sufficient lobby space to hold the expected peak accumulation of patrons waiting for retrieval of their cars. the pay stations or other access readers shall also be located inside the Lobby. Also sufficient electronic message centers shall be provided to guide the patrons to the respective location to meet their car.

In case toilets are not available in the near vicinity of the Lobby, such shall be provided in sufficient quantity to accommodate the patrons needs close to the Lobby.

The operator room shall be located near the lobby to enable the operator to assist patrons and to oversee the operation of the garage.

All areas accessible for the public in an M / A System shall be illuminated with 40 Footcandle at a height of 3 feet above finished floor.

The Terminal area further shall comply with federal and local regulations.

 

5.3   Throughput Capability

The throughput of a system is the minimum number of cars a system can store or retrieve (measured in any random one way traffic), in the timeframe of one hour. However it must be taken into consideration that the patrons drive their car into or from the Terminals. A reasonable average dwell time of 30 seconds per car driving into the Terminal can be assumed if physical drive-in guidance is provided. In the absence of such physical guidance system, an average dwell time of 60 seconds shall be considered.

 

5.4   Traffic Effects and Queuing

For the implementation of a Mechanical/Automated System with more than 30 cars capacity, a traffic study shall be required to give the system manufacturer the opportunity to address queuing issues for that particular installation. For queuing calculations, a 15 min period of peak traffic shall be compared to the capability of the system to handle vehicles during that peak period. The result will indicate the number of Terminals required and the length of vehicle queue resulting – if any- and required at each Terminal.

 

5.5   ADA requirements

As the only exchange point in a Mechanical/Automated System are the Terminals, only the access in and out of this area needs to meet the requirements of the Americans with Disabilities Act Accessibility Guidelines in addition to the other public and employee areas of the facility. These requirements include the ability to accommodate overheight vehicles (8’2’’clear) as well as a minimum 36" clear accessible route in and out of the Terminal. Alternatively a sufficient number of ADA compliant spaces can be provided outside of the Mechanical / Automated System.

 

5.6   Fire Safety

The general scope of a Building Code is, to "control all matters concerning the construction, alteration, addition, repair, removal, demolition, location, occupancy, and maintenance of all buildings and structures, and shall apply to existing or proposed buildings and structures."

The intend of the Code is "To insure public safety, health and welfare insofar as they are affected by building construction, through structural strength, adequate means of egress facilities, sanitary equipment, light and ventilation, and fire safety; and in general, to secure safety to life and property from all hazards incidents to the design, erection, repair, removal, demolition or occupancy of buildings, structures or premises."

"The provisions of the Code apply to all matters affecting or relating to structures ....and their construction, alteration, repair, addition, and removal"

Classification of an Automated Parking Facility (APF) from a classical use and occupancy viewpoint (Public Garage) would fall into ‘Use Group S-2, Low Hazard, Storage’. However, the viewpoint of a Conventional Public Garage does not apply for an APF.

The existing BOCA Code and its Commentary provides the following explanatory information regarding other Use Groups: (IBC 2000 and 2002 Supplement are derived and very similar to the 1996

BOCA in this regard)

"Buildings and structures of an accessory character and miscellaneous structures not classified in any specific use group shall be constructed, equipped and maintained to conform to the requirements of the code commensurate with the fire and life hazard incidental to their occupancy. Use Group U shall include....., tanks, cooling towers, retaining walls and buildings such as private garages, carports, sheds and agricultural buildings"

"Structures that are classified in the Use Group U, utility and miscellaneous use group, are typically accessory to another building or structure and are not more appropriately classified in another use group. .........They are not usually considered to be habitable or occupiable. Nevertheless many code provisions do apply and need to be enforced (e.g., structural loading foundations and material performance). Structures classified in Use group U are not specifically regulated by Table 503 for heights and areas because they are typically small, stand-alone structures having negligible fuel loads. Local zoning laws or ordinances will typically regulate the size of such utility structures." Further, if sections 507.1 are conformed to and NFPA 231C as listed in Chapter 35 are implemented, the “one story” facility according to use group S2 ( and type 2c [IIb] construction) does not need to meet height and area limitations.  (compare section 507.1 and exception 3 BOCA 1996)  Further explanations see chapter 5.8 in this guideline)   

Based on the results of our fire risk analysis, the following conclusions can be made:

·         The probability of a fire in an APF is low

·         Fires will usually be confined to the vehicle in which they originate. Fire spread from car to car is rare.

·         Explosion or rupture of fuel tanks as a result of a car fire is unlikely

·         Sprinkler systems are not likely to extinguish a car fire within an APF. Sprinkler systems will limit to a degree the burning of tires, exterior paint, and plastic body parts.

·         Property damage from car fires in garages is generally limited to the car of origin and loss of life and injuries are not expected, as the APF is not occupied.

·         Provisions shall be implemented in an APF that leakage of gasoline tanks or other flammable fluids are collected during transportation and storage of the vehicle.

·         The automated storage equipment and the building / structure that will house the equipment (the building / structure) shall be considered separately. The equipment shall not contribute to satisfying any of the structural requirements of the building / structure.

·         Construct the APF building / structure and the equipment with non-combustible construction without a specified fire resistance. In addition, those portions of the facility used for the transport and / or storage shall have a finish of non-absorbent, noncombustible material. Where the APF is located below a building, a 2-hour fire resistance rated separation shall be provided between the APF and the adjacent space use.

·         As the nature of an APF provides the means to transport a vehicle without human interference, provisions shall be provided to detect a vehicle on fire and to transport it to a fire extinguishing cell at a space on ground-floor, easy accessible for firefighters.

 

5.7   Sound Emissions / Vibrations

As machines are moving weights of 10 to 20 Kips, certain noise emissions are to be expected as long as the structure is the supporting element for the machinery and the building. The manufacturer knows the dBa value and needs to ensure that the surrounding walls cover any sound emission of more than 40 dBa emanating outside the building, measured at the property line.

Should the garage be part of a mixed-use complex, not only sound but also vibrations resulting from the machinery need to be considered for potential negative impact to the rest of the building. Separating the foundation and supporting members are the best means to keep the influence to a very minimum.

 

5.8     Zoning / Building Code Considerations

Zoning aspects

Mechanical /Automated Systems do not need a special code or zoning or ordinance, as all building and construction related aspects are covered by existing building codes for low hazard storage facilities. Adjustments may be made in regards of zoning requirements for slot sizes, drive aisles, ramps, turning radii and inside heights as these requirements can be reduced for Mechanical / Automated Systems. Miami Dade County in Florida made such an adjustment with the following wording, which is recommended:

START QUOTE:
"....effective immediately, off- street parking spaces including tandem parking spaces that are provided by an automated parking garage system (robotic parking garage) shall not be subject to the minimum parking size requirements nor the parking stall and aisle dimensions provided in Article VII (off-street parking) of the Miami-Dade County Zoning Code. Automated parking systems must comply with Article VII in all other respects. In an automated parking garage, the robotic parking system mechanically parks and retrieves vehicles in a multilevel parking garage structure. The driver of the vehicle is not permitted to enter the automated parking garage vehicle storage area.

Automated parking garage systems shall be permitted in zoning districts where parking garages are permissible as a principal or ancillary use, subject to compliance with the regulations of the applicable zoning district, resolution, or covenant running with the land. The establishment of automated parking garages in other zoning districts where parking garages are not allowed as a matter of right shall continue to require approval at a zoning public hearing.

Non mechanized off-street parking spaces must comply with the provisions of Article VII."
END QUOTE

Some jurisdictions have already adopted automated warehouses, storage / retrieval  and such may be applicable for Mechanical / Automated Systems as well. Regarding zoning aspects, and APF can be considered as applicable for parking- and storage regulated areas as well.

Building Code issues

The Nation’s first fully automated parking facility was permitted by the Construction Official and Sub Code Official, under the 1996 BOCA code with the following classifications:

The upcoming IBC 2000 with its 2002 supplement is very similar if not identical in this regard of permitting of an APF.

-          Use Group S2 , low hazardous according to section 311.3

-          Type 2c construction, noncombustible according to table 503

-          Section 504 allows building height modifications if an automated sprinkler system is installed in accordance with section 906.2.1

-          Section 507 indicates “unlimited areas” for one story buildings – amongst others – in use group S2, if section 507.2 is adhered to.

-          Exception Nr. 3 of section 507.1 regulates that buildings and structures of Types 1 and 2 construction for rack storage facilities, which do not have access by the public shall not be limited in height provided that such buildings conform to the requirements of section 507.1 and NFPA 231C listed in Chapter 35.

With these classifications, the APF was permitted and constructed accordingly and given the CO by the construction officials. The entire system is equipped with an automatic dry sprinkler system and respectively rated exterior walls.

5.9   Graphical User Interface / ON-Line Support

Automated Parking Facilities shall be furnished with a Graphical User Interface (GUI), or HMI, Human Machine Interface. This interface shall be positioned in the control-room. The GUI shall show the geometry of the entire System with occupancy and all installed machines moving in real time. The GUI shall be capable of running fully automated without human assistance, it shall have manual and maintenance mode and the capability of System Diagnostic of all critical mechanical, electrical and electronic equipment.

The parking facility shall have an installed and all time workable dial-in capability to the manufacturer / technical operator with a short response time to handle any alarms generated by the system.

 

5.10   Ventilation / Heating / Air Conditioning

Areas accessible to the public / patrons shall be equipped with sufficient heating, air conditioning and ventilation.

The storage area of the System in which no patrons enter but only maintenance crews, may be unheated and unconditioned space although some ventilation of outside air is recommended. No ventilation of vehicle emissions is required inside the storage area. Depending on the design of the Terminal, a ventilation of emissions may be required in that area.

In case a natural air ventilation is designed for the storage area, the air turnover ratio shall be 1.0 the volume of the entire storage volume for a 12 hour period.

 

5.11   Lighting / Accessibility for Maintenance

Lighting in areas accessible to the public / patrons shall be illuminated to a minimum of 25 Footcandles measured at 3 feet above walking surfaces.

The storage areas of the System shall be illuminated to a minimum of 1 footcandle primarily for maintenance access. Portable task lighting outlets shall be provided for enhanced lighting for repairs, diagnostics and maintenance.

The system shall be designed such, that maintenance personnel has access to all storage spaces, machinery and electrical and electronic components in a safe manner. The usage of harnesses is acceptable as long as OSHA criteria are followed.

 

5.12   Diagnostics

Automated Systems may include a Graphical User Interface (GUI), or Human Machine Interface (HMI). These devices are computer models of the layout of the garage illustrating the movement of the various transport devices within the system and are designed into the software and displayed on the computer screen.  This interface shall be positioned in the control-room. The GUI shall show the geometry of the entire System with actual occupancy and all installed machines reflecting real time movements / positions of machinery and stored vehicles.

The GUI, based on the source code shall be capable of running fully automated without human assistance; it shall have manual mode and maintenance mode operations and the capability of System Diagnostic of all critical mechanical, electrical and electronic equipment components. 

 

The parking facility may have an installed and real time, on-line connection to the manufacturer / technical operator which allows for resolution of most errors remotely with a short response time in reaction to any trouble alarms generated by the system.

 

The system shall be capable of reporting alarms in different classes according to their severity for the System functionality. It shall report the need of component checks for proper performance of the different components. Preferably, a hotline support line shall be implemented to enable a remote system support.

5.13  Maintenance / Operator

 

In order to ensure the operational safety, planners, manufacturers and owners shall coordinate and implement suitable measures in order to avoid System interruptions as much as possible and to remedy such interruptions in a reasonable time. 

A preventative maintenance schedule shall be in place covering all critical system components to be maintained for proper functionality. The regular scheduled maintenance of the System by qualified personnel; as well as on-site spare parts are indispensable measures.

In case of public use of the APF, trained personnel must be available at short notice. The manufacturer undertakes to provide a trained technician for the prevention and remedy of interruptions during the main hours of operation of the APF. To cover the remaining hours of operation, the manufacturer shall train owner’s on-site personnel as “system officer” to the degree, that these “officers” are capable of retrieving a car in semi automatic mode of operation.

APF’s that do not serve public users may be staffed with “system officers” only. 

The System diagnostics as mentioned above shall report to the on-site technical operator about necessary maintenance performance or component checks in advance.

The storage areas of the System shall be illuminated to a minimum of 1 footcandle primarily for maintenance access. Portable task lighting outlets shall be provided for enhanced lighting for repairs, diagnostics and maintenance.

The system shall be designed such, that maintenance personnel has access to all storage spaces, machinery and electrical and electronic components in a safe manner. The usage of harnesses is acceptable as long as respective safety regulations are followed.

5.14  Spare Parts Package

Enough spare parts shall be stored at the facility to ensure an immediate availability of exchange components in any case of a component failure.

In order to ensure needed repairs to be performed in a timely manner, the manufacturer and owner shall agree for a respective maintenance contract, covering the operating times of the Facility including a sufficient on-site spare parts package. A response time for System generated alarms of 30 minutes or less is recommended.