Parking Lots - Studies and Design

Parking studies must be conducted to collect the required information about the capacity and use of existing parking lots. 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 minimising 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 summarised by time period for the entire study area. The occupancy can be calculated by taking accumulation/total spaces.

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 trade-off 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.

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

T = 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, and M = 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 maximise the number of spaces provided, while allowing vehicles to park with only one distinct manoeuvre. 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 realise 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 manoeuvres would you need to make? Knowing that, what needs to be included in the design to make sure all those manoeuvres are possible? Use the following list of manoeuvres 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).

Parking Stall Layout Considerations:

The objective of the layout design is to maximise 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 mini vans 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.

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.