Lab Exercise One: Facility Analysis and Design A local off-street parking facility might be inadequate. You have been asked to evaluate the adequacy of the parking facility. If your results show that the facility is inadequate, you are to propose suitable modifications and/or additions that will correct the problem. You should prepare a brief report that summarizes your analysis and any proposed modifications to the facility. Your instructor will designate the off-street parking facility and provide any additional information that might be needed. Tasks to be Completed Task 1: Develop an inventory of the parking facility including: the location, condition, type, and number of parking spaces, any time limits, hours of availability and other restrictions, and the geometry of the spaces and other features. Task 2: Estimate the peak parking period for the facility and complete an accumulation count study for the facility during that period. Using your results, develop an accumulation graph. Task 3: Perform a simplified license plate survey and estimate the average length of time that vehicles are parked at the facility during the peak parking period. Task 4: Using the information from the first three tasks, calculate the probability that an incoming car will not find a parking space during the peak parking period.
Transcript
Lab Exercise One: Facility Analysis and Design
A local off-street parking facility might be inadequate. You have been asked to evaluate the adequacy of the parking facility. If your results show that the facility is inadequate, you are to propose suitable modifications and/or additions that will correct the problem. You should prepare a brief report that summarizes your analysis and any proposed modifications to the facility.
Your instructor will designate the off-street parking facility and provide any additional information that might be needed.
Tasks to be Completed
Task 1: Develop an inventory of the parking facility including:
the location, condition, type, and number of parking spaces, any time limits, hours of availability and other restrictions, and the geometry of the spaces and other features.
Task 2: Estimate the peak parking period for the facility and complete an accumulation count study for the facility during that period. Using your results, develop an accumulation graph.
Task 3: Perform a simplified license plate survey and estimate the average length of time that vehicles are parked at the facility during the peak parking period.
Task 4: Using the information from the first three tasks, calculate the probability that an incoming car will not find a parking space during the peak parking period. Determine whether or not the probability of rejection is acceptable.
Task 5: If the current parking facility is inadequate, design modifications and/or additions that will correct the problem. Your goal is to suggest the most economical solution. Report any assumptions that you make.
Parking Studies
Studies must be conducted to collect the required information about the capacity and use of existing parking facilities. In addition, information about the demand for
parking is needed. Parking studies may be restricted to a particular traffic producer or attractor, such as a store, or they may encompass an entire region, such as a central business district.
Before parking studies can be initiated, the study area must be defined. A cordon line is drawn to delineate the study area. It should include traffic generators and a periphery, including all points within an appropriate walking distance. The survey area should also include any area that might be impacted by the parking modifications. The boundary should be drawn to facilitate cordon counts by minimizing the number of entrance and exit points.
Once the study area has been defined, there are several different types of parking studies that may be required. These study types are listed below and discussed in detail in the remaining paragraphs.
Inventory of Parking Facilities Accumulation Counts Duration and Turnover Surveys User Information Surveys Land Use Method of Determining Demand
Inventory of Parking Facilities:
Information is collected on the current condition of parking facilities. This includes:
the location, condition, type, and number of parking spaces. parking rates if appropriate. These are often related to trip generation or
other land use considerations. time limits, hours of availability and any other restrictions. layout of spaces: geometry and other features such as crosswalks and city
services. ownership of the off-street facilities.
Accumulation Counts:
These are conducted to obtain data on the number of vehicles parked in a study area during a specific period of time. First, the number of vehicles already in that area are counted or estimated. Then the number of vehicles entering and exiting during that specified period are noted, and added or subtracted from the accumulated number of vehicles. Accumulation data are normally summarized by time period for the entire study area. The occupancy can be calculated by taking accumulation/total spaces. Peaking characteristics can be determined by graphing the accumulation data by time of day. The accumulation graph usually includes cumulative arrival and cumulative departure graphs as well.
Above Figures taken from:Khisty, C. and M. Kyte, Lab and Field Manual for TransportationEngineering, Prentice Hall, Englewood Cliffs, NJ, 1991.
Duration and Turnover Surveys:
The accumulation study does not provide information on parking duration, turnover or parking violations. This information requires a license plate survey, which is often very expensive. Instead, modifications are often made to the field data collection protocols. Note that there is usually a tradeoff between data collection costs and study accuracy. Spending more time and money may increase
accuracy, but at what point does the incremental change in accuracy become too expensive?
In planning a license plate survey, assume that each patrolling observer can check about four spaces per minute. The first observer will be slower, because all the license plate numbers will have to be recorded, but subsequent observers will be able to work much faster. The form shown below can be used for a license plate survey.
Parking turnover is the rate of use of a facility. It is determined by dividing the number of available parking spaces into the number of vehicles parked in those spaces in a stated time period.
Table1. Typical License Plate Survey Field Form for Curbside Survey
Street_______Side ____Study Date_________________ Data Collector_________From_____________________ To ________________ Direction of Travel ___________
Space No
Space Desc.
Time at beginning of Patrol8:00 8:30 9:00 9:30 10:00 10:30 11:00 11:30
User Information Surveys:
Individual users can provide valuable information that is not attainable with license plate surveys. The two major methods for collecting these data are parking interviews and postcard studies. For the parking interviews, drivers are interviewed right in the parking lot. The interviews can gather information about origin and destination, trip purpose, and trip frequency. The postage paid postcard surveys requests the same information as in the parking interview. Return rates average about 35%, and may include bias. The bias can take two forms. Drivers will sometimes overestimate their parking needs in order to encourage the surveyors to recommend additional parking. Or, they may file false reports that they feel are more socially acceptable.
Land Use Method of Determining Demand:
Parking generation rates can be used to estimate the demand for parking.
Tabulate the type and intensity of land uses throughout the study area. Based on reported parking generation rates, estimate the number of parking
spaces needed for each unit of land use. Determine the demand for parking from questionnaires. A rule of thumb is
to overestimate the demand for parking by about 10 %. If the analysis suggests that the parking demand for a particular facility will be 500 spaces, then the design should be for 550 spaces.
Adequacy Analysis
The adequacy of a parking facility can be measured by calculating the probability that an entering vehicle will not be able to find a parking space. A high probability of rejection (not finding a space) may indicate that expansion of the parking facility is warranted.
The probability of rejection can be calculated by comparing the traffic load to the number of parking stalls as shown below.
First, the traffic load is estimated using:
A = Q*T
Where:A = traffic load,Q = incoming vehicle flow rate, andT = the average parking duration.
Make sure that your units of time cancel each other. If you give Q in vehicles per hour, then use T in units of hours.
Next, calculate the probability of rejection using the following formula:
P = (AM/M!)/(1 + A + A2/2 + . . . + AM/M!)
Where:P = the probability of rejection,
A = the traffic load, andM = the number of parking stalls.
If the probability of rejection is high, you may want to consider adding more parking stalls to the parking facility.
Parking Facility Design Process
The goal in designing off-street parking facilities is to maximize the number of spaces provided, while allowing vehicles to park with only one distinct maneuver. It would be nice to present a step-by-step procedure for reaching this goal, but it isn’t that simple. Parking lot design requires balancing a variety of concerns. For example, you might decide on a nice layout for your parking lot, only to realize that you haven’t provided any spaces for persons with disabilities. The next iteration would correct this error, but might very well create another problem. You simply have to hammer out all of the kinks, until you end up with a design that satisfies all of your criteria.
One way to start is to imagine that you are parking your own car in a lot. What maneuvers would you need to make? Knowing that, what needs to be included in the design to make sure all those maneuvers are possible? Use the following list of maneuvers to guide your thinking.
1. Vehicle enters from street (space provided by entry driveway).2. Vehicle searches for a parking stall (space provided by circulation and /or
access aisles).3. Vehicle enters the stall (space provided by the access aisle).4. Vehicle is parked (stall designed to accommodate the vehicle’s length and
width plus space to open vehicle doors).5. Pedestrians access the building or destination (usually via the aisles).6. Vehicle exits the parking stall (space provided by the access aisle).7. Vehicle searches for an exit (space provided by the access and circulation
aisles).8. Vehicle enters the street network (space provided by the exit driveways).
Entrance Considerations
The first maneuver that a parking vehicle will make involves leaving the street and entering the off-street parking lot. This maneuver, while simple, requires some careful thought by the parking lot designer.
Analysis of the demand for parking may indicate that there are periods during the day in which a large number of vehicles want to enter the parking facility at roughly the same time. The entrance to the parking lot must be able to handle the entering traffic without forcing vehicles to wait in the street, because stagnant vehicles will reduce the capacity of the adjacent street. To avoid conflicts with other traffic, entrances should be located as far from intersections and conflict points as possible. Multiple entrances may ease access and reduce restriction on the adjacent roadways.
Years of experience have produced general entrance dimensions that seem to work. The basic nominal design width for a two-way driveway serving commercial land use is 30 ft., with 15 feet radii. With greater volumes such as at a shopping center, a 36-ft driveway may be appropriate. It should be marked with two exit lanes (each 10 or 11 ft wide) and a single entry lane (14-16 ft wide) to accommodate the off-tracking path of entering vehicles. Larger commercial facilities such as regional shopping centers may require twin entry and exit lanes separated by a 4-12 foot median.
Many areas of the country have specific regulations or guidelines for the design of access facilities. It is important that the local and state regulations concerning access management are followed when designing access to off-street parking facilities.
Internal Considerations
There are two major internal maneuvers that the parking lot designer must consider—vehicles searching for an open stall and vehicles searching for an exit. These internal maneuvers require space, which is space that cannot be used for parking.
Off-street parking facilities normally operate in one of two ways. The first and most common operation is ‘self-parking’, in which the driver maneuvers the vehicle through the parking lot. The second operation is ‘attendant parking’, in which parking attendants maneuver the vehicle through the parking lot. Parking facilities that use ‘self-parking’ must normally include larger aisles, as individuals unfamiliar with the parking facility may require extra room to maneuver. Attendant parking is normally more expensive to operate than self-parking.
Tollbooths and other restrictions at entrances or exits also affect the internal operation of parking facilities. Tollbooths require reservoir space within the parking facility for vehicles that are waiting. In general, 2-3 spaces per lane are required at entrances to self-parking lots where a ticket needs to be acquired. At exits, a much larger reservoir should be provided, because toll collection requires extra time.
Finally, in areas where winter snowfall is common, consideration of the snowfall removal operation should be included in the design process. Adequate space must be provided for snow removal equipment to maneuver.
Parking Stall Layout Considerations
The objective of the layout design is to maximize the number of stalls, while following the guidelines below.
The layout of the parking facility must be flexible enough to adapt to future changes in vehicle dimensions.
The stall and aisle dimensions must be compatible with the type of operation planned for the facility.
The critical dimensions are the width and length of stalls, the width of aisles, the angle of parking, and the radius of turns. All of these dimensions are related to the vehicle dimensions and performance characteristics. In recent years there have been a number of changes in vehicle dimensions. The popularity of minivans and sport utility vehicles has had an impact on the design of parking facilities. For the near future, a wide mix of vehicle sizes should be anticipated. There are three approaches for handling the layout:
1. Design all spaces for large-size vehicles (about 6 feet wide and 17-18 ft long).
2. Design some of the spaces for large vehicles and some for small vehicles (these are about 5 ft wide and 14-15 ft long).
3. Provide a layout with intermediate dimensions (too small for large vehicles and too big for small vehicles).
For design, it is customary to work with stalls and aisles in combinations called "modules". A complete module is one access aisle servicing a row of parking on each side of the aisle. The width of an aisle is usually 12 to 26 feet depending on the angle at which the parking stalls are oriented.
Stall Width
For simplicity, the stall width is measured perpendicular to the vehicle, not parallel to the aisle. If the stall is placed at an angle of less than 90o, then the width parallel to the aisle will increase while the width perpendicular to the vehicle will remain the same.
Stall Length
The length of the stall should be large enough to accommodate most of the vehicles. The length of the stall refers to the longitudinal dimension of the stall. When the stall is rotated an angle of less than 90o, the stall depth perpendicular to the aisle increases up to 1 foot or more. It should be noted that the effective stall depth depends on the boundary conditions of the module, which could include walls on each side of the module, curbs with or without overhang, or drive-in versus back-in operations. For parking at angles of less than 90o, front bumper overhangs beyond the curbing are generally reduced with decreasing angle and, for example, drop to about 2 feet at 45o angles. The Table 8-3 below gives the standard dimensions for several different layouts as defined by Figure 8-4.
Table 8-3 and Figure 8-4 where taken from:Weant, R.A. and Levinson, H.S., Parking, Eno Foundation, 1990, page 161.
Interlock Module
A special type of module, the interlock, is possible at angles below 90o. There are two types of interlock. The most common, and preferable, type is the bumper-to-bumper arrangement. The second type, the "herringbone" interlock, can be used at 45o and is produced by adjacent sides having one way movements in the same direction. This arrangement requires the bumper of one car to face the fender of another car. Figure 8-3 shows several different module layouts that are commonly used.
Comparing Angle Efficiencies
The relative efficiencies of various parking angles can be compared by looking at the number of square feet required per car space (including the prorated area of the access aisle and entrances). Where the size and shape of the tract is appropriate, both the 90o and the 60o parking layouts tend to require the smallest area per car space. In typical lot layouts for large size vehicles, the average overall area required (including cross aisles and entrances) ranges between 310 and 330 square feet/car. A very flat angle layout is significantly less efficient than other angles.
One-Way Aisles
There are many conditions where one-way aisles are desirable. With parking angles less than 90o, drivers can be restricted to certain directions. However, the angle should usually be no greater than 75o. Drivers may be tempted to enter the parking aisles and stalls from the wrong direction when the stall angle is too large. Adjacent aisles generally have opposite driving directions.
Exit Considerations
The last maneuver that a vehicle will make in a parking facility involves leaving the facility and entering the adjacent street network. Inefficiencies in this part of the parking process can lead to reduced capacity in both the parking facility and the adjacent street network.
As was discussed in the section entitled ‘Entrance Considerations,’ access and egress points should be located as far as possible from any conflicting points on the adjacent street network. This normally means that entrances and exits are placed ‘mid-block.’ Multiple exit lanes may be required, so that right-turning vehicles can avoid waiting for left-turning vehicles at the exit. If the flow rate of departing vehicles is low, or if the adjacent street is one-way, a single lane may be sufficient.
The type of parking facility also impacts the exit design. Facilities that have tollbooths near the exits will require multiple exit lanes. They may also require that a large portion of the parking lot be devoted to lanes for vehicles waiting to pay at the tollbooths.
ADAAG Requirements
The Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities (ADAAG) specifies the number and dimensions of accessible parking spaces. Where possible, the accessible parking spaces should be provided on the accessible path to the facility entrance and also minimize the distance traveled.
Total Parking in Lot Required Minimum Number Of Accessible Spaces
1 to 25 126 to 50 251 to 75 376 to 100 4100 to 150 5151 to 200 6201 to 300 7301 to 400 8401 to 500 9501 to 1000 2 percent of total
1001 and over 20 plus 1 for each 100 over 1000
One in eight accessible spaces, but not less than one, should be "van accessible". These spaces should be 96 in (2440mm) wide. Parking access aisles need to be part of the accessible path to the building. Two adjacent accessible spaces may share a common access aisle. The access aisle should be 5 feet wide. Parking spaces and access aisles should be level, with surface slopes not greater than 1:50 (2%) in all directions.
Design of Off-Street Facilities
The following excerpts were taken from the1992 edition of theTraffic Engineering Handbook, published by the Institute of Transportation Engineers (pp. 205-215).
Elements of Good Design (pp. 205-206)
In designing any off-street parking facility, the elements of customer service, convenience, and safety with minimum interference to street traffic flow must receive high priority. Drivers desire to park their vehicles as close to their destination as possible. The accessibility, ease of entering, circulating, parking, unparking, and exiting are important factors. Good dimensions and internal circulation are more important than a few additional spaces. Better sight distances, maneuverability, traffic flow, parking ease, and circulation are the results of well-organized, adequately designed lot or garage.
Site Characteristics
Factors such as site dimensions, topography, and adjacent street profiles affect the design of off-street parking facilities. The relation of the site to the surrounding street system will affect the location of entry and exit points and the internal circulation pattern.
Access Location
External factors such as traffic controls and volumes on adjacent streets must be considered --- particularly the location of driveways or garage ramps. It is desirable to avoid locating access or egress points where vehicles entering or leaving the site would conflict with large numbers of pedestrians. Similarly, street traffic volumes, turning restrictions, and one-way postings may limit points at which entrances and exits can logically be placed. It is important to investigate these factors at the beginning of design.
Driveways should be located to provide maximum storage space and distance form controlled intersections. . . .
General Elements and Layout Alternatives (p. 212)
Because of their lack of walls or cover, parking lots have no ventilation problems, and lighting is sometimes provided by relatively tall poles, thus affording high efficiencies and minimizing the number of poles. Generally, lots have clear sight lines and offer a feeling of greater security than in a more confined space. Lots are not restricted on vehicle heights and thus afford access to both commercial and emergency vehicles. . . .
Generally, the layout of a parking lot seeks to strike a balance among maximizing capacity, maneuverability, and circulation. . . .
The general advantages of 90 parking, as compared with lesser angles, are:
1. Most common and understandable;2. Can sometimes be better fitted into buildings;3. Generally most efficient if site is sufficiently large;4. Uses two-way movement (can allow short, dead-end aisles);5. Allows unparking in either direction. Thus it can minimize travel distances and
internal conflict;6. Does not require any aisle directional signs or markings;7. Wide aisles often provide room to pass vehicles stopped and waiting for an unparking
vehicle;8. Wide aisles increase separation for pedestrians walking in the aisle and between
moving vehicles;9. Wide aisles increase clearance from other traffic in the aisle, during unparking
maneuvers;10. Fewer total aisles (hence easier to locate parked vehicle).
Several advantages and disadvantages of angle parking (usually 45 to 75 ), are:
1. Easiest in which to park2. Can be adapted to almost any width of site by varying the angle;3. Requires slightly deeper stalls but much narrower aisles and modules;4. Drivers must unpark and proceed in original direction; hence producing greater out-
of-way travel and conflict;5. Unused triangles at end of parking aisles reduce overall efficiency;6. To avoid long travel, additional cross aisles for one-way travel are required, which
adds to gross area used per car parked;7. Difficult to sign one-way aisles.
Wheel Stops and Speed Bumps (p. 215)
In general, the ends of parking stalls within lots can be marked in a satisfactory fashion by only a paint line. Wheel stop blocks in the interior of a lot have disadvantages, for they may interfere with and present a hazard to people walking between cars, provide traps for blowing debris, and interfere with snow plowing in northern climates. . . .
Wheel stops are often used along the side boundaries of a lot, where large landscaped areas extend beyond the edge of pavement and an occasional override would present no significant hazard.
Adequacy Analysis
Over the course of an 8-hour day, 96 vehicles enter a local electronics store’s parking lot. The parking lot has 5 spaces and the average customer stays in the grocery store for 15 minutes. Calculate the probability that an incoming car will be rejected.
Solution
First, we need to calculate the incoming flow rate. This is done as follows:
Q = 96 vehicles/ 8 hoursQ = 12 vehicles/hour
Since we know the average vehicle is parked for 15 minutes, or 0.25 hours, we can calculate the traffic load as follows.
Each entering vehicle has an 11% chance of being rejected. As a result, the electronics store loses one out of each 10 customers entering their lot.
new sandwich shop is nearing completion and a parking lot needs to be designed. The storeowners anticipate that, on the average 12-hour day, 360 vehicles will visit the sandwich shop. The owners also anticipate that the average vehicle will remain parked for 10 minutes. How many parking spaces need to be provided in order to guarantee that no more than 1 vehicle in 50 will be unable to find a parking space?
[Solution S
Solution
First, we need to determine the traffic load. The incoming flow rate is calculated as shown below.
Q = 360 vehicles/12 hoursQ = 30 vehicles/hour
The average parking duration is 10 minutes or 0.167 hours. The traffic load is calculated as shown below.
A = 30 vehicles/hour * 0.167 hoursA = 5
The maximum probability of rejection is 1 in 50, or 0.02. Using the probability of rejection equation, we can solve for the number of spaces required.
P = (AM/M!)/(1 + A + A2/2 + . . . + AM/M!)
Where:P = the probability of rejection (0.02),A = the traffic load (5), andM = the number of parking stalls.
Solving the equation for M yields a value of 10. The parking lot at the sandwich shop must have at least 10 spaces, in order to meet the owner’s expectations.
Note that we have used average parking rates in this analysis. The sandwich shop’s particular situation could dictate that more spaces are required. For example, say that the shop serves 80% of its customers between 11 A.M and 2 P.M. The majority of the customers are arriving during a much shorter time frame than the 12 hours that we used to find the incoming flow rate. In this case, more parking spaces would be required.
There are many sources of information about Parking Lot Design on the Internet. You might want to further your education by taking advantage of these resources.
National Transportation Library: Parkinghttp://www.bts.gov/ntl/subjects/parking.html
Parking Today magazine, which features the parking industryhttp://www.parkingtoday.com/
Seattle Metro/King County WA travel management website http://www.metrokc.gov/kcdot/transit/
Use your browser’s ‘back’ button to return to the previous page.
Accessible Path-- a barrier-free path that persons with mobility or sensory impairments can safely follow without obstacles or obstructions. Accessible paths are at least 5 feet wide and level.
Aisle-- the portion of the parking lot devoted to providing immediate access to the parking stalls. The recommended aisle width is dependent on the parking angle. A parking angle of 45 o requires an aisle width of 12 feet for a 9.0-foot stall, and a 90o parking angle requires an aisle width of 26 feet for a 9.0-foot stall. These dimensions lead to wall to wall distances of 47 feet for 45o and 63 feet for 90o.
CBD-- Central Business District, typically ranging from an average size of 27 blocks (10,000-25,000 population cities) to over 200 blocks (cities over 1,000,000 population).
CBD Core-- the heart of business, commercial, financial and administrative activity. Typically ranges in size from an average of 7 blocks (10,000-25,000 population cities) to over 60 blocks (cities over 1,000,000 population).
CBD Fringe-- the area immediately surrounding the CBD, usually within 2-3 blocks.
Change of Mode-- the transfer from one form of transportation to another. A park and ride lot is an example of a change of mode, where an auto driver parks the vehicle and rides public transportation for the remainder of the trip.
Cordon Count-- the simultaneous counting of all traffic entering and leaving a given area such as a CBD. It is generally a manual vehicle classification count, supplemented with automatic traffic recorder counts.
Duration-- the length of time a vehicle remains in one parking space.
Long Term Parking-- parking with a duration of three hours or more.
Module-- a complete module is one access aisle, servicing a row of parking on each side of the aisle. Both the access aisle and the parking stalls serviced by that aisle are part of the module.
Outlying Business District-- commercial area generally removed by a mile or more from a central CBD.
Parking Accumulation-- the total number of vehicles parked in a specific area (usually segregated by type of parking facility) at a specific time.
Parking Demand-- the number of vehicles with drivers desiring to park at a specific location or in a general area. It is usually expressed as the number of vehicles during the peak-parking hour.
Parking Space or Stall-- an area large enough to accommodate one parked vehicle with unrestricted access (no blockage by another parked vehicle).
Parking Supply-- the number of spaces available for use, usually classified by on-street curb (metered and unmetered), lot and garage. Further differentiation of the types of parking is useful, such as those available to the general public, and private spaces earmarked for a specific purpose such as loading.
Parking Volume-- the total number of vehicles that park in a study area during a specific length of time.
Partial Module-- one access aisle combined with a single one-side row of parking.
Short Term Parking-- parking with a duration of three hours or less.
Stall Length-- The longitudinal dimension of the stall, normally 18.5 feet.
Stall Width-- The width of each parking space as measured crosswise to the vehicle. The most common width is 8.5 to 9.0 feet.
Study Period-- the time during which the parking study is conducted, usually between 10:00 A.M. and 6:00 P.M. Increasing emphasis, however, is being placed on inclusion of the morning and evening periods within the length of the study. Certain uses, such a theatres, may peak in the evening hours, while residential parking demand peaks around 3:00 A.M.
Trip Purpose-- the primary reason for the individual’s journey to the study area. Typical purposes include shopping, working, business, and recreation.
Turning Radii-- The radius of the circle that is traveled by the design vehicle when completing a turn. Large turning radii should be provided. These are a function of the parking angle and end island design, but in general the turning radii should be at least 18 feet.
Turnover-- the number of different vehicles parked at a specific parking space or facility during the study period. Parking turnover measures utilization.
Van Accessible-- a parking space that is at least 8 feet wide, with a minimum access aisle of 5 feet along the right side of the parking space.
Parking Stall Layout Considerations
The objective of the layout design is to maximize the number of stalls, while following the guidelines below.
The layout of the parking facility must be flexible enough to adapt to future changes in vehicle dimensions.
The stall and aisle dimensions must be compatible with the type of operation planned for the facility.
The critical dimensions are the width and length of stalls, the width of aisles, the angle of parking, and the radius of turns. All of these dimensions are related to the vehicle dimensions and performance characteristics. In recent years there have been a number of changes in vehicle dimensions. The popularity of minivans and sport utility vehicles has had an impact on the design of parking facilities. For the near future, a wide mix of vehicle sizes should be anticipated. There are three approaches for handling the layout:
1. Design all spaces for large-size vehicles (about 6 feet wide and 17-18 ft long).
2. Design some of the spaces for large vehicles and some for small vehicles (these are about 5 ft wide and 14-15 ft long).
3. Provide a layout with intermediate dimensions (too small for large vehicles and too big for small vehicles).
For design, it is customary to work with stalls and aisles in combinations called "modules". A complete module is one access aisle servicing a row of parking on each side of the aisle. The width of an aisle is usually 12 to 26 feet depending on the angle at which the parking stalls are oriented.
Stall Width
For simplicity, the stall width is measured perpendicular to the vehicle, not parallel to the aisle. If the stall is placed at an angle of less than 90o, then the width parallel to the aisle will increase while the width perpendicular to the vehicle will remain the same.
Stall Length
The length of the stall should be large enough to accommodate most of the vehicles. The length of the stall refers to the longitudinal dimension of the stall. When the stall is rotated an angle of less than 90o, the stall depth perpendicular to the aisle increases up to 1 foot or more. It should be noted that the effective stall depth depends on the boundary conditions of the module, which could include
walls on each side of the module, curbs with or without overhang, or drive-in versus back-in operations. For parking at angles of less than 90o, front bumper overhangs beyond the curbing are generally reduced with decreasing angle and, for example, drop to about 2 feet at 45o angles. The Table 8-3 below gives the standard dimensions for several different layouts as defined by Figure 8-4.
Table 8-3 and Figure 8-4 where taken from:Weant, R.A. and Levinson, H.S., Parking, Eno Foundation, 1990, page 161.
Interlock Module
A special type of module, the interlock, is possible at angles below 90o. There are two types of interlock. The most common, and preferable, type is the bumper-to-bumper arrangement. The second type, the "herringbone" interlock, can be used at 45o and is produced by adjacent sides having one way movements in the same direction. This arrangement requires the bumper of one car to face the fender of another car. Figure 8-3 shows several different module layouts that are commonly used.
Comparing Angle Efficiencies
The relative efficiencies of various parking angles can be compared by looking at the number of square feet required per car space (including the prorated area of the
access aisle and entrances). Where the size and shape of the tract is appropriate, both the 90o and the 60o parking layouts tend to require the smallest area per car space. In typical lot layouts for large size vehicles, the average overall area required (including cross aisles and entrances) ranges between 310 and 330 square feet/car. A very flat angle layout is significantly less efficient than other angles.
One-Way Aisles
There are many conditions where one-way aisles are desirable. With parking angles less than 90o, drivers can be restricted to certain directions. However, the angle should usually be no greater than 75o. Drivers may be tempted to enter the parking aisles and stalls from the wrong direction when the stall angle is too large. Adjacent aisles generally have opposite driving directions.
