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C H ~ :A P T E R 1 0FUNCT IONALDESIGN OFPARK INGSTRUCTURESH . Carl W alker and Jam es E. Sta if

A Parking structure is part of the roadway system where vehicle occu-pants change from the vehicular mode to the pedestrian mode andback again. Every vehicle requires at least two parking spaces-thefirst at its trip origination point and the second at its destination. The needfor parking structures is created by people driving to places of concentratedactivity-a central business district, shopping center, airport, sports facility,college campus, medical center, or office building. The functional design of aparking facility varies with the type of user. A parking facility serving a retailshopping center or medical center experiences a high daily turnover of park-ing space use while a parking structure serving an office building may havea single turnover unless additional demand is created during evening andweekend hours.

Functional design involves the development of vehicular and pedestrianflows in a parking structure as well as the layout of parking spaces. Parkingfacility operation, including revenue control and security, is another function-al design consideration.Types of Parking Structure OperationsThe two general types of parking operations are self-park and attendant -parkfacilities. In attendant-park facilities, the driver leaves the vehicle at theentrance and an attendant parks the vehicle. When the driver returns, theattendant retrieves the vehicle and transfers it to the driver at the exit. Atten-dant-park facilities maximize the number of vehicles that can be placed in aparticular area, usually through stacked parking (see Figure 10-1). Such facili-ties are often associated with high-value properties in larger metropolitan areas.

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FIGURE 10-1ATT EN DA NT -PA RK FA CIL ITY

r: 11 II[II II

1:11 II ~ II IIII II

1:11 III

Today, the most common type of parking operation inNorth America is the self-park facility, in which drivers parkand retrieve their own vehicle (see Figure 10-2). This chapterdiscusses the self-park approach to functional design.

The elements of functional design to be considered forself-park structures include the following: adjacent street traffic analysis;

location and number of entrances and exits;FIGURE 10-2

SElF -PA RK F ACIL ITY

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FIGURE 10-3

MORE TH AN ONE EN TRANCE/EXIT

Two-WayIr-~I ( transient versus monthly parking; large-vehicle/small-vehicle ratios; vehicle and pedestrian circulation; parking space geometries; parking space resizing provisions; allowances for future expansion; stair and elevator locations; lighting intensity and control; drainage; security systems; access and revenue control and operating systems; graphics and signage, and maintenance and durability factors.

Street TraHicStreet traffic configuration-the pattern of adjacent one-and/or two-way streets-has a major impact on the use of;Jparking structure. It is always best to locate entrances onmajor streets inbound to the destination area, with exitslocated on outbound streets. The greatest fear of a parkingfacility owner is a change in the one-way street pattern aftera garage is completed. Such a change requires the immedi-ate reversal of all access directions to and from the facility'sentrances and exits. The location of parking structure exitson low-volume streets is recommended as a means of mini-mizing street traffic conflicts with vehicles exiting from theparking facility.

EntrancesGenerally, entrances to a parking facility are located on thehigh-volume streets that provide direct access from users'

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ParkingStructure

ParkingWo. -..oil- Decelerat ion

lan e .Through-Traff iclanes

1p.tsof origin. Larger parking facilities generally incor-;iate more than one entrance and exit to provide access

various adjacent streets in the event of street repairs orencies (see Figure 10-3). Moreover, should entranceting equipment fail, the additional entrance lane pro-alternative access. As one solution to the problem of.

ntial equipment failure, some operators place two ticket, nsers in tandem at a single entry lane and cover onea removable canvas hood.It is generally more convenient to enter a facility from a-.way street or by turning right from a two-way street. leftinto a garage from a major two-way street during peakc periods can be difficult, if not impossible, because ofhigh volume of traffic approaching from the opposite

ion.The architectural design of entrances is a major element

_uccessful facility operation. Entrances must be designedbe obvious and should be differentiated from facility exits.ial architectural features such as arches, canopies, mar-

-,~, or other distinguishing design elements help users~ntify a facility's entrance(s).

Where a parking facility is adjacent to a high-volume or. -velocity street, a deceleration lane leading to the entrance_ps eliminate rear-end accidents and reduces street trafficowdowns (see Figure 10-4). An entrance lane is usuallyipped with a ticket dispenser, access card reader, control

;pte, and loop detectors (see Figure 10-5). An access cardreader can be added to a typical entry lane but should beIaced upstream of the ticket dispenser so it is not mistakenr a ticket dispenser. When installed in combination withn access card receiver, a ticket dispenser should be a push-

FIG URE 10 -5 - - -- - -T ICKET D ISPENSER

K ey C ardReceiver

FIGURE 10-6MON TH LY /H OU RLY PA RK IN G

o

loop\ Detector\ Gote

KeyCardReceiver

fMonthlyContractPark ing

_'-.:.i.

TicketDispenser

fHourlyPark ing

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FIGURE 10-7

D IM EN SIO N AL C L EA RA N CES

0 -J \Ticket 0Jo oDispenser

;;-- ~ ~G

Q)uc :: :.Q'"(5E

~: :: :JEc~ - , ..I 1 -" I\ .. ~

c_ Jbutton unit so that entrance operations and activity reportingremain separate functions. Separate lanes can be designatedfor monthly and hourly parking (see Figure 10-6).

Entrance ticket dispensers and gates should be located farenough in from the street so that a vehicle entering behind avehicle already at the ticket dispenser or access card readercan clear the sidewalk. When a deceleration lane cannot beaccommodated on high-traffic streets, ticket-dispensing andcard-reading equipment should be located at least three orfour vehicle lengths into the building to provide queuingspace. A large, van-type vehicle should be used as the designvehicle to test dimensional clearances (see Figure 10-7).

One inbound lane is adequate for a garage with anaverage turnover of 300 to 500 vehicles. For larger garages(500 or more vehicles) or smaller garages with high turnover,additional entrances or entrances on different streets helpaccommodate the higher entering volumes. A traffic opera-tions assessment of the adjacent streets can determine thebest possible access points. Entrances should be located atleast 75 to 100 feet from any comer intersection to preventconflicts between parking facility traffic and street intersec-

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Key CardReceiver

C A SH IER B OO T HFIGURE 10-8

, ,

, I\ j

Cashier~+f--Booth'--_ . . . o Gate---. . .-

\ l- Exit QueueC""'--_

tion traffic. Traffic engineering principles can be applied todetermine the appropriate vehicle queueing storage length.

Single entrance lanes from the street should be approxi-mately 13 to 16 feet wide, tapering down to ten feet at theapproach to the control equipment. Double entrance lanesshould be a minimum 24 feet wide. Ramps leading directlyfrom the entrances should have a short, level segment beyondthe controlled exit/entrance area before beginning the-slope,upward or downward to the parking spaces.Operators often use television cameras to monitorentrances that are remote from the cashier booth or manag-er's office. The cameras, coupled with an audio communica-tion system built into the ticket dispenser, aid in communica-tion if an equipment malfunction occurs or a patron has aquestion before entering the facility.

ExitsIf possible, exits should be located on low-volume streets toreduce any exiting delay caused by street traffic in the vicini-ty of the exit. In addition, a turn in the drive aisle leading toan exit lane or plaza can slow exit speeds and help control

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e release rates into the street. It is preferable to grouper all cashier booths so that one cashier can operatearking structure during low-volume periods, thus mini-g operating costs. The number of cashier-staffed and-card exit lanes varies with the ratio of monthly con-

"t (access card)' patrons to transient parkers."For a typical municipal garage with a combination ofi ue nt and monthly parkers, one cashier-staffed lane forh 400 exiting vehicles is usually adequate, with a mini-of two lanes for a structure accommodating at least

vehicles. One lane is the primary cashier-staffed exit; ther lane is a secondary or peak-load cashier-staffed lane.ddition, the second lane is available for monthly parker-card exiting and therefore allows the monthly parker

ypass any backup that may occur at the adjacent cashierth oTo keep cashiering delays to a minimum, the cashierth should be located far enough from the sidewalk ort to provide space for at least one vehicle that hasdy completed the parking payment transaction (see Fig-

"10-8).A cashier can handle approximately 90 to 180 vehicleshour depending on the simplicity of the parking rates andtype of revenue control equipment. For example, parkingscharged in dollar increments permit vehicles to moveugh the cashier plaza faster than rates requiring coin

ange. Sometimes the primary cashier booth is an extension ""the manager's office. In off-peak periods, the same staff

n can serve the dual function of shift manager andhier.perations Systemsost self-park facilities use a n exit cashiering system; howev-, some operate with parking meters located at each space. at centralized pedestrian exits. Sometimes a flat fee is col-cted upon entry. Operations systems can vary from simpleetered systems to highly complex and sophisticated remotemputerized systems. Other systems such as automatic pay--foot and central cashiering are common in Europe and. . g acceptance in North America.

ehicle Circulation PaHerns e most common circulation pattern used in parking struc-es in North America is the continuous ramp, wherebyoping floors with aisles and parking spaces along both'des of the aisle provide access to the parking spaces them-lves and the facility's circulation route; The basic continu-us-ramp, sloping-floor configuration is called the single-elix or scissors ramp. It is used for 90-degree parking ando-way traffic (see Figure 10-9).Architects generally like functional designs that provide

_ orizontal and vertical facade lines. They often object" toJa~ades with sloping lines. Thus, functionai designs that

keep the ramping on the interior bays, such as the three-bay double-helix ramp or the four-bay side-by-side ramp,are preferred.

Several systems use combinations of variations on thesingle-helix continuous ramp: the two-bay, end-to-end con-figuration (see Figure 10-10); the two-bay double-threadedhelix configuration (see Figure 10-11); the three-bay, double-threaded configuration (see Figure 10-12); and the four-bay,side-by-side configuration (see Figure 10-13). All of theseconfigurations lend themselves to one-way traffic and angleparking.

The two-bay split-level configuration (see Figure 10-14)was once a commonly used layout. Its geometry, however,requires special interfloor ramps that are not needed in con-tinuous-ramp systems, thus leading to higher structural costsas well as to poor traffic circulation.

While 9O-degree parking could be used with a one-waytraffic system, it does not lend itself to the "automatic" one-way traffic flow pattern created by angle parking (see Figure10-15). More specifically, 9O-degree parking is frequently asso-ciated with the two-way traffic patterns that result in cross-traffic or conflict points within the structure, longer parkingspace search times for the driver, and larger overall buildingwidth. In contrast, the advantages of angle parking includeease of entry into a parking space and minimization of two-way traffic conflicts.

Nonetheless, 9O-degree parking should not be dismissedoutright. A 9O-degree, two-way traffic pattern can sometimesoperate much as a one-way traffic layout. In particular, afacility with high inbound traffic in the morning and highoutbound traffic in the evening, as in the case of an employ-ee parking facility, may effectively operate as a one-way traf-fic facility.

Access from floor to floor is typically provided by slop-ing one or more bays from one level to the next level.Multiple-bay sloped ramps can easily confuse the uninitiatedparking patron. Where repeat parkers represent the largestshare of facility patrons, however, users come to understandthe ramp configuration. Under such circumstances, the effi-ciency and shorter travel distances afforded by intertwining "ramps, such as with the double-threaded helix configuratien,are particularly desirable.For'facilities that serve the infrequent parker at, for exam-ple, a convention center or hospital, the parking layout shouldbe as simple as possible. In this case, the preferred practiceis tq replicate the shopping center type of parking lot byspecifying level floors and, at the more remote bays, contin-uous-ramp parking that accommodates vehicle ingress/egress.

In a continuous-flow circulation system, drivers shouldnot have to pass more than 600 to 750 spaces in a drivingcircuit to locate a parking space. The design should also per-mit a driver leaving a parking space to circulate directly to"the exit as expeditiously as possible. A reentry point to thefacility's internal flow pattern is desirable and usually located

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FIGURE 10-9S IN GL E-H ELIX R AM P (TWO-W A Y TR AF FIC )

on the ground floor or first supported level before the entryto the cashier lanes.Typically, grades in continuous-ramp facilities do notexceed 5 to 6 percent on the parking floors. For nonrepeti-tive short segments or in hilly areas where people are accus-tomed to steep grades, parking floor grades up to 7 percentare acceptable. In any event, the needs of persons with dis-abilities must be considered in establishing parking floorgrades. Speed ramps (nonparking ramps) should be limitedto a 12.5 percent, grade unless signage specifically prohibitspedestrian use of the ramps. Ramps greater than 15 percentcan be psychological barriers to some drivers, particularly inthe case of a downbound ramp; however, again, in hillyareas, ramp grades up to 20 percent may be considered.

FIGURE 10-10END-TO -END HELIX RA MP (O NE-W AY TRA FFIC)

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FIGURE 10-11TWO-B AY , D OU BL E-TH RE AD ED H EliX R AM P(O NE -WAY TRA FF IC )

FIGURE 10-12THREE -BAY, DOUBLE -THREADED HEL IX RAMP(O NE -WAY TRA FF IC )

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FIGURE 10-13

FO UR-BAY , S IDE -BY -SIDE RAMP (ONE-W A Y TRA FF IC )FIGURE 10-15

PERPEN DIC ULA R A ND A NGLE PA RKIN G

W- { o o e - w a y ~

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FIGURE 10-17H EIG H T C L EA RA N C E

TrueVehicle Clearance

= =~--~Floor to Ceiling

ly 15 feet wide with an outer-curb width of 18 inches and aninner-curb width of 12 inches. Sometimes, circular ramps aredesigned in a double-threaded configuration in which eachcomplete turn marks a descent of two parkin~ levels.

The grade on a circular ramp varies with floor-to-floorheight, the width of the driving path, the number of floorsper revolution, 'and the overall diameter of the ramp. Thegrade also varies from the outer circumference to the innercircumference. All turns on the ramp should be a continuousramp slope and superelevated between six inches and onefoot with no reverse superelevation. Circular ramps shouldb,~as open as possible. Solid walls on the inner and outer~'eumferences of the ramp create a sense of confinement

thus should be discouraged. A circular express rampld be limited to six complete turns, which in the case ofuMe-threaded ramp would allow the ramp to serve a

- .stoty structure.

- .-ng Space and Module Design- si efficient parking structure in terms of square feet

is not necessarily developed with 90-degree park-i Properly designed angle-parking layouts with one-

ssovers can sometimes be more efficient than 90-Q~ts. In fact, the ease of parking in a one-way traf-- t often offsets any decrease in efficiency.", g space width and parking module lengthfrom bumper wall to bumper wall) can benee with the desired level of comfort. Aster 8, some parking structure designers'aces for large and small vehicles; howev-nded parking space calls for one size fits-arking spaces now tend to be 8 feet, 6d with large-vehicle spaces of nine feetspaces of 7 feet, 6 inches wide.

The ratio of large vehicles to small vehicles varies withdifferent locations in North America as well as with differentuses. For example, average vehicle size in the West is gener-ally smaller than that in the Midwest. In addition, the aver-age vehicle size at universities, particularly among student-driven vehicles, is smaller than average vehicle sizes at' otherlocations. Often, a simple vehicle-size study at a given loca-tion can provide the parking facility designer with an accu-rate picture of the vehicle-size mix in that area. Currentdevelopments in designing for different vehicle sizes are dis-cussed in chapter 8.

Local zoning codes and the Americans with DisabilitiesAct require accessible parking spaces for vehicles driven bypersons with disabilities (see chapters 4, 8, and 17 for a !poredetailed discussion of zoning, geometries, and the Americanswith Disabilities Act). The incorporation of special design fea-tures for the disabled is mandatory illnewly designed facili-ties. In some cases, designated spaces may have to complywith requirements for increased clearance and a prescribedlocation(s) within the parking structure.

Some parking consultants use a level-of-service conceptto specify the size of parking spaces. Level of Service (LOS)as used in highway design ranges from LOS A for free, unim-peded flow of traffic to Levels B, C, D, E, and, finally, LOS Ffor gridlock. In fact, these considerations have little applica-tion to parking structure functional design. Other parking con-sultants, recognizing that parking functional design requiresevaluation factors that differ from those used in roadway/highway design, prefer the User Comfort Factor approach todesign. The User Comfort Factor approach takes into accountvariables such as one- or two-way traffic, 90-degree versusangle parking, parking space width, drive aisle width, queu-ing space available at entrances and exits, cashier systems,driver choices while circulating, and potential vehicle/vehicleand vehicle/pedestrian conflicts. ~t should also be noted thatlevel of service and user comfort are improved by increasingmodule or space width.

Another consideration in sizing parking spaces is climat-ic factors. Given the inclement winter months in northernstates, it may be desirable to specify a slightly wider spacethat permits drivers to exit from and enter their vehicleswithout their heavy outerwear making contact with adjacentvehicles that might be splattered with road chemicals andmud.

Clear-Span DesignThe design of parking structures to span the parking moduleand thus eliminate columns between parked vehicles offersseveral advantages. First, clear spans promote easy entry intoparking spaces without the "fender-bender" stigma. Second,columns between parking spaces consume space that couldotherwise be used for parking. Most important, however, theclear span allows for the future restriping of parking spacesas vehicle sizes change.

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en that column locations seldom interfere with park-rations in clear-span structures, the structural bay sizeot need to be identical to a specified increment ofg space width. In other words, if 9O-degree parkingare laid out at 8 feet, 6 inches on center, the columnshave to be placed at 25 feet, 6 inches on center or atother increment of 8 feet, 6 inches. Columns can proj-, the parking module by up to two feet. While. today'sger vehicles vary considerably in length, some largemay-but rarely do-park adjacent to the projecting,and extend into the parking aisle. Even then, they

-ot seriously disrupt a facility's operations.

estrian Circulationtravel paths are typically located behind parked

les along the sides of drive aisles; the exception to this. special pedestrian routes that may be required for theled. In either case, pedestrians require a protected pathcess to and from the parking structure and a safe, visi-'.ute to their parking space. Vertical transportationlties for pedestrians such as elevators and stairs intend-rpatrons' use are usually located at the perimeter ofarking structure in the direction of major destinations.

_escape exit stairs must be located in accordance withapplicable building code. Facilities with a high rate ofle turnover, which means high pedestrian volume,t incorporate wider drive aisles. Elevator lobbies and'towers may be the most visible element' of a parkingre and may provide interior architectural designrtunities for incorporating glass, special lighting, brick,ornamental features. '

ding code requirements dictate the exit travel distance toftre -exits. If stairs are the only means of vertical pedes-conveyance, as in a building of one or two supportedIs, at least one stair should be oriented to major destina-. In parking facilities with four or more parking floors,

. locations may not be critical except in meeting buildinge requirements. In multilevel facilities, most patrons useelevators, although one stairway should be located adja-t to the elevator tower. If the elevator is out of order,ons use the nearby stairs. Even when an elevator israting, some patrons prefer to walk to lower floors.The general design standard is one elevator for up to 250

aces, two elevators for up to 500 spaces, and three to fourevators for up to 1,000 spaces, although the number of lev-

, 1 s , rate of vehicle turnover, and type of land use supportedy the parking facili ty can be factored into the standard.

Common elevator size in aparking facility is a 2,500-pound-capacity unit with a ftve-foot-by-seven-foot cab thatan accommodate a gurney for medical evacuation. Except.'in the case of low-rise hydraulic elevators, the vertical speed

should be no less than 200 feet per minute. Hydraulic eleva-tors can be used in structures up to six levels.

Where high peak-load pedestrian traffic is projected,extrawide stairs are recommended to encourage patrons' useof the stairs during periods of high activity.

Other ConsiderationsIf a parking structure is enclosed or located undergroundand thus does not meet the open-structure wall area require-ments for natural ventilation, mechanical ventilation andsprinklers may be required. Building codes usually prescribethe rate of ventilation. Generally, a rate of one cubic foot perminute per square foot of area is adequate for a parkingstructure except where extensive interior queuing is expect-ed. Carbon monoxide sensors are recommended for under-ground facilities. /

When required, sprinkler systems are insta ed in a con-ventional manner. In freezing climates, a dry stem is pref-erable. A dry standpipe with hose cabinet is ometimes usedinstead of a sprinkler system.

Overall experience shows that the like' ood of fire in aparking structure is extremely low. Nonethel ss, it should benoted that ftre extinguishers located in a parking facility posesome risk of theft. Fire extinguishers stored in, the cashierbooth and manager's office usually should suffice for fireprotection purposes in most cases.

Curbs in the vicinity of stair and elevator towers are triphazards and thus should be discouraged. Often; curbs areused as wheelstops when facili ty design calls for cable railsinstead of concrete bumper walls. Curbs between parkingmodules on a flat parking deck are also discouraged. Patronsmay trip over them r-ey move from bay to bay betweenvehicles.

Precast concrete wheelstops are not recommended. Theyprovide an area where trash and debris collect. If, however,wheelstops are required with angle parking, they should beplaced in a straight line rather than perpendicular to the park-ing space. When wheelstops are placed perpendicular to theparking space, the wheelstop in the adjacent space poses atrip hazard for the driver exiting from a vehicle in that sPace.The hazard is particularly problematic with vans because thedriver sits unusually close to the front of the vehicle.

The efficient functional design of a parking facility isa critical component in the success of a facility. Interactionbetween pedestrians and vehicles and between vehiclesshould be provided in a safe and efficient manner. Vehicularand pedestrian circulation within the facility and to and fromthe facility needs to be accommodated with appropriatelydesigned entrances, exits, parking bays, elevators, and safetyfeatures. This will help ensure higher use levels, lower liabil-ity, and an efficient and cost-effective facility for the user andowner.

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