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Estimating Parking Demandfor Mixed-Use DevelopmentsSubject to TSM OrdinancesBY GEOK K. KUAH

und, accura te park ing -demand es-timates for new developments are ofconcern not only to developers, but alsoto public officials, nearby residents, andtraffic engineers. Developers are con-cerned because their decision on thenumber of parking spaces to provide canaffect their ability to get a loan to financetheir projects; lending institutions oftentake the position that inadequate or sub-standard parking facilities can reduce aprojects marketability, thereby threat-ening the success of a project.The developer, however, wants to keep

costs down by providing just enoughspaces for prospective tenants. Providingadditional spaces will drive costs up,either for land or for an additional levelof underground parking. Developers re-alize the importance of providingenough parking spaces for new develop-ment at the time the project is built be-cause adding more parking spaces to afinished building at a later date can bedifficult.The residents of communities where

new developments are planned are alsoconcerned about parking. Typically, res-idents of nearby, already establishedcommunities oppose new developmentsif they feel that these developments will

Conversion Factor To convert from ~ multiply bySqft m2 ).0929

add to the congestion on neighborhoodstreets. If these citizens fail to stop thedevelopment altogether, they demandthat developers provide enough parkingto accommodate traffic introduced bythe development so that traffic does notspill over onto their neighborhoodstreets.Public officials fear that the abun-

dance of free parking will deter com-muters from using transit systems andwill hinder the successful implementa-tion of transportation systems manage-ment TSM) programs. As stated byCervero, the ease of finding free spaceson site acts as a strong incentive to travelby private automobile and a disincentiveto use other transportation modes orprograms.Traditionally, a developments parking

demand is calculated based on parking-code requirements as stipulated in cityor county parking ordinances. Theseparking ordinances are developedmostly to regulate parking supply formeeting peak parking demand at single-use developments. Many jurisdictionshave established minimum and maxi-mum parking-code requirements for newdevelopments. The minimum require-ment ensures there will be adequatespaces available, with the maximum re-quirement limiting the number of spacesso as to enhance community objectives,such as promoting the use of alternatetravel modes.The calculation of a projects parking

demand based on code requirements is

complicated by the recent passage omunicipal and county ordinances aimedat reducing peak-period, single-occu-pancy vehicle trips through the use oTSM programs and shared parking ordinances for mixed-use developmentsMXDS). For example, MontgomeryCounty, Maryland, and the City of Alexandria, Virginia, have sections in theiordinances dealing with parking reduc-tions/credits for specific uses participat-ing in TSM programs and for MXDSsharing parking spaces. zs These new ordinances, like many other ordinances,provide only general parking-require-ment guidelines and do not specify thactual parking needs for individualMXDS. Estimates of parking demandbased on code requirements alone usually will not satisfy developers.Traffic engineers working for devel

opers frequently must justify at publichearings their estimates of the numberof parking spaces needed for a projectand any deviations from the numberspecified in the code requirements. Theengineers are often asked by developersto produce more accurate demand estimates.It is clear that a sound procedure fo

estimating parking demand is essentialto developers, public officials, the publicand traffic engineers. It is particularlyuseful in situations where the tenantdensity deviates substantially from thaverage density for a specific use or icases where a deviation from the regularparking code requirement is pursued.

ITE JOURNAL . FEBRUARY 1991.19


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The procedure for estimating parkingdemand is complex. It involves many fac-tors, including project size, type of zon-ing, type and number of persons ex-pected to visit the site, availability ofalternative transportation modes, andthe time frame of the analysis.Most of the existing literature on es-

timating parking demand deals primarilywith single-use projects.- Literatureon estimating parking demand forMXDS is scant, and no literature to datehas dealt with parking-demand estima-tion for MXDS subject to TSM ordi-nances or the interactions betweenshared parking and parking reductionsbecause of TSM programs.Some traffic engineers use the regres-

sion-based method, as documented inITEs Parking Generation, to estimateparking demand, while others follow thecode requirements. This diversity hascreated problems for city/county staffsresponsible for reviewing traffic reportssubmitted by developers and for grantingapproval for the requested parkingspaces.This article proposes a methodology

for estimating parking demand forMXDS planned in jurisdictions withTSM ordinances. Unlike the regression-based method, x which estimates park-ing demand for specific land uses by per-forming regression analysis on parkingoccupancy data collected across thecountry, the proposed method is a proj-ect-specific, user-based approach thatextends the concept discussed byWeant . It accounts for potential parkingreductions resulting from the implemen-tation of TSM programs and the sharingof parking spaces for MXDS.

Analytical ApproachThe approach consists essentially ofthree steps. First, it estimates the basicparking demand for employees and non-employees for each of the proposed usesof a MXD. Next, it reduces the basicparking estimates to account for the pro-posed TSM programs. Finally, the re-duced parking demand is adjusted to ac-count for shared parking amongindividual land uses of the development.The basic parking demand for employ-

ees related to employee density, absen-tee and visitor factors, average auto-oc-cupancy rate, and mode split isestimated as shown in Equation 1.

D=

WhereD=E=

T =

V =Yofl =

(E- Ex A-Ex T)/O+ Ex?Zo VE 1YoA T)10+Ex V [1]

Basic parking demand for theMXD,

Estimated number of employeesfor the proposed MXD,

Number of employees usingnonauto modes to work as apercentage of total employ-ees,

Number of visitors from othersites as a percentage of totalemployees,

Percentage of total employeesabsent on an average day, and

Average auto occupancy for theem~loyee before-TSM.

The absentee factor includes sick em-ployees and employees absent from thesite to attend meetings, etc. TSM pro-grams can reduce parking demand. Typ-ical TSM programs available to privatedevelopers are transit, ride-sharing, andstaggered work hours. Both transit andride-sharing programs have the potentialto reduce parking demand by reducingthe number of vehicle trips coming intothe project. As more employees usetransit, the percentage of nonauto worktrips 70 T) increases; thus, parking de-mand decreases. Likewise, as more em-ployees share rides to work, the em-ployee auto-occupancy rate increases;thus, parking demand decreases.Staggered work-hour programs

spread peak-hour traffic loads andthereby reduce peak-hour traffic vol-umes, but they will not reduce parkingneeds because the vehicles that arriveoutside the peak periods will still requireparking spaces. Based on Equation 1, parking demand

for the MXD with a proposed TSM pro-gram isD, = E I A)/0, E( T,/0,)

+Ex70V [la]Similarly, parking demand for the MXDwithout a proposed TSM program isD, = E I A/02) E TJOJ

Ex V [lb]Thus, parking reductions stemming froma proposed TSM program can be calcu-lated by subtracting Equation lb fromEquation la.

D, D, = E[ l A) 1/0, 1/0,) T,IO, YoT,/O,)]

[2]WhereD, =D, =

T, =T, =o =02 =

Parking demand for the MXDwith a proposed TSM program,Parking demand for the MXDwithout a proposed TSM pro-gram,Nonauto mode splits before theTSM program,Nonauto mode splits after theTSM program,The average auto-occupancy ratebefore the TSM program, andThe average auto-occupancy rateafter the TSM program.

As shown in Equation 2, the term asso-ciated with visitor parking demand dropsoff because visitor parking demand is anonemployee parking demand and is notaffected by the employee TSM program.Next, the reduction in parking demandas a result of shared parking is calcu-lated. MXDS create opportunities forshared-parking arrangements that canreduce parking demand of a project.The reduction is made possible by thephenomenon that parking demands fordifferent land uses of an MXD peak atdifferent periods during the day.Many citieslcounties have shared-

parking provisions in their ordi -nances.2 ~ For citieslcounties withshared-parking provisions, the availableparking accumulation schedule could beused to calculated the amount of allow-able parking reduction. For cities/coun-ties without shared-parking ordinances,the parking accumulation schedule doc-umented in Shared Parking, supple-mented with survey data collected lo-cally on similar projects within the samejurisdiction, could be used. Shared-parking reduction should be

considered only if the shared-parking fa-cilities are under the same ownershipand control and are located within a rea-sonable walking distance 500750 feet)of various single-use establishments ofthe MXD. Furthermore, to estimate thedegree of shared parking, parking de-mand adjusted for TSM programs, in-stead of the unadjusted basic parking de-mand, should be used with theaccumulation schedule when calculatingthe amount of parking reduction result-ing from shared parking. The data re-

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Table 1. Data Requirements and SourcesData Type Data SaurceDevelopment Program DeveloperEmployee/Nonemployee Densi ty Developer or Cify /CounfyMode Sp iit be fo re TSM Cify/Counfy or SurveyMode Spil t a fter TSM Cify /Counfy and TSM OrdinanceVehic ie Occupancy Cify/Counfy or SurveyAbsenteeism Rate Cify/Counfy, Developer, or Survey/0 Visitors City/County, Developer, or SurveyParking Accumulation Schedule Shared Parking Ordinance, Survey, or Existing

Literature,

Table 2. Estimating Number of Employees and NonemployeesLand Use Sze Ratio No. of Personsmp loyeeOffice Emp loyee 1,000,000 GSF 3.5 Employees/1000 GSFRetail Empioyee 80,000 GLA 4 Employees/1000 GLAHotel Emplayee 150 Rooms 0.4 Empioyees/ RoomTOTALonemp loyeeHotei Guest 150 Rooms 1,4 Guesfs/ RoomRetaii Shopper 80,000 GIA 4 Shoppers/1000 GLAResident 500 DU 2,2 Residents/ DUTOTAL

Tota i No. Employees and Nonemployees

350032060

3880

210320110016305510

GSF = Gross Square FeetGLA= GrossLeaseableAreaDU = Dwelling Units

quirements and their sources used in theproposed parking-demand estimationprocedure are summarized in Table 1.

An ExampleThe following example illustrates theproposed method for estimating parkingdemand for MXDS iocated in a jurisdic-tion having TSM ordinances. Consider aplanned MXD project located within themetropolitan planning district of theCity of Alexandria, Virginia, for whichthe parking demand needs to be esti-mated. The development consists of 1million square feet of office develop-ment, 80,000 square feet of gross leasa-ble area GLA) of retail development, a100-room hotel, and 500 dwelling unitsof residential development.The groups of users for the proposed

MXD include employees from office,hotel, and retail facilities; hotel guests;retail shoppers; and residents. Under

Alexandria ordinances, a TSM programfor the development is required to re-duce single-occupancy vehicle SOV),peak-period trips by 1030 percent, de-pending upon its location within theplanning district. Assuming the requiredreduction is 30 percent for the proposedMXD, a TSM program proposed by thedeveloper could consist of various incen-tives to attract transit ridership and par-ticipants in ride-sharing and staggeredwork-hour programs to achieve the tar-get SOV trip reduction.In estimating parking demand for the

MXD, the following steps are used:1. Identify the various groups of usersof the MXD,

2. Estimate the total number of employ-ees and nonemployees,

3. Estimate the unadjusted parking de-mand using Equation 1) for eachuser group,

4. Estimate the number of person trips

5.

6.

7.

8.

and vehicle trips by mode with andwithout the TSM program,Calculate the average employee ve-hicle-occupancy rate with and with-out the TSM program,Determine parking reductions result-ing from the proposed TSM programfor employees,Determine parking reductions asso-ciated with shared parking using theparking accumulation schedule, and,finally,Subtract the parking saved as a resultof the TSM program and of sharedparking from the unadjusted parkingdemand to obtain the adjusted park-ing demand.

As shown in Table 2, the total estimatednumber of employees and nonemployeesdetermined according to the develop-ment program and the user densities are3880 and 1630, respectively. Using avail-able mode-split data obtained from thecity, employee person trips and vehicletrips by mode and average vehicle occu-pancy without the proposed TSM pro-gram are calculated and are presented inTable 3. If existing mode-split data arenot readily available, employee travel-to-work surveys should be conducted forsimilar projects located in the same ju-risdiction.The unadjusted parking demand for

employees after considering absentee,visitor, mode-split, and vehicle-occu-pancy factors is 3880 1 10 11 )/1.057 + 3880 x 59Z0= 3094 spaces,calculated using Equation 1, as shown inTable 4. Similarly, for nonemployees, thedemand is 1269 spaces, as shown in Ta-ble 4. Typically, nonemployees, such asretaii shoppers, residents, and hotelguests, have different mode-split and ve-hicle-occupancy characteristics relativeto employees. These data needed to beestablished through local surveys and/orliterature searches. The total unadjusteddemand without the TSM program is4363 spaces, which is the sum of totalemployee and nonemployee demandTable 4). The average vehicle-occu-pancy rate, including carpool and van-pool vehicles, is calculated as 1.06 seeTable 3).The proposed TSM program Table 3)

anticipates a 7 percent increase in transitmode share, a 5 percent increase in van-pool mode share, a 7 percent increase incarpool mode share, and a 3 percent in-



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Table 3. Number of Person Trips and Average Vehicle Occupancy for All EmployeesVWhout TSM With TSM

Existing No . o f Average Future No. of AverageMode Person Vehicle No . o f Mode Person Vehic le Na . o f

Mode Splits 7.) Trips Occupancy Auto TriPs Splits ) Trips Occupancy Auto TripsBus Only 3 116 NA NA 5 194 NA NAMetrorail 5 194 NA NA 10 388 NA NACommuterRail 1 39 NA NA 1 39 NA NAWalk +Bicyc le 2 78 NA NA 2 78 NA NA

Vanp oo l 8 0 0 8 0 5 194 8 24Ca rpoo l 3+ 2 78 3 26 5 194 3 65Ca rp oo l 2 6 233 2 116 10 388 2 194SWH 5 194 1.1 176 9 349 1.1 3~7Sov 76 2949 1 2949 53 2056 1 1056Tatal 100 3880 3267 100 3880 2657

Nonauto Mod e = 11 Nonauto Mode = 18A ve ra ge Vehic le Oc c up anc y = 1.057 A ve ra ge Vehic le Oc c up anc y = 1.198

Reduc tio n in SOV TriDs = [2949 20561/2949 = 307.TSM = Transportation Systems ManagementSWH = Staggered Work HoursSOV = Single-Occupancy VehicleNA = Not Applicable

Table 4. Unadjusted Parking Demand for Employees and NonemployeesWithout TSM With TSM

User No , o f Absentee Visitor Nonauto Vehic le Parking Nonauto Vehicle ParkingCategory Users ) ) Mode ] Occupancy Demand Mode Yo) Occupancyb DemandmployeeOffice 3500 2794 2279Hotel 320 255 208Retail 60 48 39Subtotal 3880 10 5 11 1.06 3094 18 1,19 2527onemp loyeeHotelGuest 210 0 0 5 1,40 143 5 1,40 143

RetailShopper 320 0 0 10 1.25 230 40 1.25 230

Resident 1100 0 10 0 1.40 896 0 1.4 896Subtotal 1630 1269 1269TOTAL 5510 4363 3795Pa rking Red uc tion Due to TSM = 3094-2527 = 567 Sp ac esb Vehicle uccupancy and percentage of nmrvehicle mode split are obtained from Table 2.TSM = Traff ic Systems Mmragement

crease in staggered-work-hour-program for example, a TSM program with a SOV trip reduction. Thus, it is importantmode share. In order to meet the re- higher percentage of transit mode share to assess the effectiveness of the pro-quirements of the City of Alexandria, and a lower percentage of ride-sharing posed TSM program in achieving the an-developers are allowed to create a TSM mode share than the one suggested in ticipated percentage increase in modeprogram to meet the reduction target in Table 3 will result in a larger reduction share for each travel mode contained inSOV trips. The reduction can be in the number of parking spaces and the program.achieved through a number of means; achieve the same number of the required Table 3 shows a low auto-occupancy

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Table 5. Adjustment tor Shared ParkingParking Accumulation Schedule [?fo) No. of Spaces by Hour

Hours Offic e Hofel Refail Residentia l Offic e Hofel Reta il Residentia l Tofal6 A.M. NA 100 NA 100 NA 351 NA 896 42467 A M 20 90 5 90 456 315 13 806 15918 A M 60 65 20 80 1368 228 54 717 23669 A M 90 55 30 70 2051 193 81 627 2952If) A.M. 100 45 50 70 2279 158 135 627 3199II A M 1 35 70 60 2279 123 189 537 312812 P M 90 30 75 60 2051 105 202 537 2896I PM 90 30 75 60 2051 105 202 537 28962 P M 1 35 75 60 2279 123 202 537 31423 P M 95 35 70 60 2165 123 189 537 30144 P M 75 50 65 70 1709 475 175 627 26875 P M 50 60 60 80 1140 210 462 717 22286 P M 20 70 60 90 456 245 162 806 16697 P M 15 75 70 95 342 263 189 851 16458 P M 15 90 65 100 342 315 475 896 47299 P M 5 95 40 100 114 333 108 896 145110 PM o 100 20 100 0 351 54 896 1301Tota lSpa ce sby Use 2279 351 269 896

Pe ak Ac cumula tion Hour = 10 A MTota l No o f Spa ces before Sha ring = 3795Tota l No o f Spaces a fler Sharing = 3199Spac es Saved as a Resulf of Shared Parking = 596 Based on City of Al cxancfr ia, Vi rgi ni a, Shared Parking Ordi nance.NA = Not Applicahlc.

value of 1.1for staggered-work-hour em-ployees because they are less likely toride-share with others. With the pro-posed TSM program, the employee ve-hicle-occupancy rate is calculated as1.198 and the percentage of employeesusing nonauto modes is increased to 18percent.The number of parking spaces saved

resulting from the TSM program basedon Equation 2) is3880 [ 1 0.1) 1/1.057 1/1.198)- 0.11/1.057 - 0.18/1.198)]= 567 spaces.

Table 4 shows a breakdown of the total567 saved spaces among various modesof the TSM program. The parking needsfor both employees and nonemployeesafter the TSM program is 3795 spaces.Because nonemployees are not subjectto the TSM ordinance, the reduction inparking spaces will come solely from theemployees.Finally, the parking accumulation

schedule see Table 5) from the citysshared-parking ordinance is used to de-termine the degree of shared-parkingpotential of the proposed MXD. The

parking demand by land use prior toshared-parking arrangements is 2279spaces for office, 351 spaces for hotel,269 spaces for retail, and 896 spaces forresidential uses. As shown in Table 5,parking spaces saved because of sharedparking is 596.The adjusted parking demand of the

project is 3199 spaces 4363 567 596), compared with the unadjusted de-mand of 4363 spaces, representing a sav-ings of 27 percent. Once the parking de-mand for a development project isaccurately determined, its parking re-quirement can be established by addinga small buffer to the estimated demand;the accepted buffer varies from jurisdic-tion to jurisdiction, and a typical valueof 5 percent can be used.

ConclusionsAs discussed, accurate parking demandestimates are vital concerns among pri-vate and public decisionmakers, the res-idents in areas where the MXD isplanned, and traffic engineers workingfor developers. This article presents aproject-specific, user-based approach to

estimate parking demand for MXDSplanned in areas subject to TSM oral;nances.The methodology not only takes into

account parking reductions because ofTSM programs, but it also addresses thesaving of spaces because of shared park-ing among different land uses of theMXDS. Savings in the number of park-ing spaces from TSM programs isachieved through a reduction in automode share and an increase in vehicle-occupancy rate. Savings in the numberof parking spaces resulting from sharedparking is realized because parking de-mand for different land uses peaks atdifferent periods of the day.Based upon the approach, developerswill be able to provide an adequate num-

ber of parking spaces that might varyfrom the code requirements. The ap-proach is particularly useful for devel-opers of projects having different park-ing needs than those stipulated in theparking codes.eferen es1. Cervero, Robert. Suburban Gridlock.Piscataway, N. J,: Center for Urban Policy



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2.

3.

4,

5.

6.

7.

Research, Rutgers University, 1986 .C it y of A l exan dr i a P ar k in g O rd i na nce.Alexandria, Va.: City of Alexandria,1988.Zoning Ordinance of the Maryland-Washington Regional District in Mont-gomery County,Urban Land Institute and National Park-ing Association. The D im en si on s ofParking, 2d Edition. Washington, D. C.:Urban Land Institute and National Park-ing Associat ion, 1983,Kershner, D. L., R. A, Makofski, and R.C. Rand. Estimating the Demand forParking in Atlantic City. TransportationQuar ter ly ctober 1981): 589-608.Institute of Transportation Engineers.Park ing Genera t ion , 2d Edi tion. Wash-ington, D.C.: lTE, 1987.Weant, Robert A. Par k i ng Gar age P lan -n i n g a nd Oper a ti on . Westport , Corm.:Eno Foundation for Transportation,1978.

8.

9.

10.

11,

12.

Urban Land Insti tute. Empl oymen t a ndPa rk i ng i n Su bu rba n B usi n e,r s Par k s: AP i lot S fu d y. Washington, D. C.: UrbanLand Institute, 1986.Kanaan, George E, Par k i ng and Acce.ma t Gen er a l H osp i ta l . Westport, Corm,:Eno Foundation for Transportation,1973.Whitelock, Edward M. Par k i ng f or I n st i -t ut ion an d Speci al E ven ts. Westport,Corm.: Eno Foundation for Transporta-tion, 1982.Urban Land Institute. Shar ed Par k i ng.Wash ington, D. C.: Urban Land Insti-tute, 1983 ,Slade, Lewis J., and G. K. Kuah. Trans-portation Systems Management Pro-gmms for Private Developers. In Com-pen di um of T ech ni cal Paoer s. 58thAnnual Meeting, pp. 397-401. Washing-ton, DC.: Institute of TransportationEngineers, 1988. I

Geok K. Kuah is aprincipal trans-portation engi-neer with DeLeuw,Cather Com-pany in New YorkCity. Prior to join-

ing DeLeuw, Cathe~ he served asdirectorof research for GorovelSlade Associates.He received a Diplom-Ing. (FH) fromStuttgart, West Germany, an M. S. C.E.from the University of Michigan, and aPh. D. from the University of Maryland.He is a registered professional engineer inMaryland and the chairman of the ITETechnical CounciI Committee ParkingGate Capacities. Kuah is an AssociateMember of ITE.

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