With over 25 years of experience planning parking lots and garages, I utilize proven design standards to maximize capacity and circulation efficiency. This article provides best practice recommendations for configuring parking area layouts tailored to specific needs while meeting American Association of State Highway and Transportation Officials (AASHTO) guidelines.
Understand Facility Usage
The first step is gathering data on expected demand and patterns:
- Peak parking occupancy – determines total spaces needed
- Typical dwell times – impacts entry/exit sizing
- Vehicle mix – dimensions spaces appropriately
- Traffic flow patterns – arrange ingress/egress and circulation
Thoroughly characterizing facility usage allows for optimizing the design accordingly.
Determine Appropriate Dimensions
With AASHTO standards as a starting point, parking stall dimensions should suit prevalent vehicle sizes:
Vehicle Type | Width | Length |
---|---|---|
Compact Cars | 8 ft | 16 ft |
Mid-size Cars | 8.5 ft | 17 ft |
Full-size Cars | 9 ft | 18 ft |
SUVs and Pickups | 10 ft | 20 ft |
Accessible Spaces | 13 ft | 18 ft |
Aisle widths likewise correlate to parking angle:
Angle (degrees) | 1-Way Aisle | 2-Way Aisle |
---|---|---|
0° Parallel | 12 ft | 20 ft |
30° | 11 ft | 20 ft |
45° | 13 ft | 21 ft |
60° | 18 ft | 23 ft |
90° Perpendicular | 24 ft | 24 ft |
Sizing for the predominant vehicle mix ensures adequate clearances.
Choose Parking Angle
The parking angle significantly impacts layout efficiency:
- Perpendicular (90°) – Best overall utilization but wide drive aisles
- Angled (45-60°) – Narrower aisles with modulated stall depths
- Parallel (0°) – Inefficient footprint but allows curb access
Usage should guide angle selection to maximize capacity.
Configure Circulation Aisles
One of the most critical parking lot layout considerations is configuring the vehicle circulation aisles. As a civil engineer with over 25 years of expertise in planning parking facilities, I carefully evaluate one-way versus two-way aisles to balance convenience, safety, and efficiency.
One-Way Aisles
One-way aisles simplify navigation, as drivers only need to watch for vehicles approaching from one direction as they cruise the lot looking for an open spot. This allows lanes to be narrower. However, one-way travel can also increase the distances and time required to get to a desired space or exit compared to a more direct two-way flow.
I typically specify one-way aisles for simple small lots with clear routing. The disadvantages become more apparent in larger lots with many aisles, where one-way circulation forces greater travel to reach proximate destinations.
Two-Way Aisles
Two-way aisles allow vehicles to directly access any space from either direction. This provides excellent convenience and flexibility for patrons familiar with the lot. However, allowing vehicles to pass in opposite directions necessitates wider lane widths for safety.
Two-way flow is preferable in larger lots with high occupancy and multiple aisles where patrons would otherwise have to circle much farther to reach nearby parking spots. However, head-on turning movements must be considered.
Balancing Tradeoffs
I aim to configure aisle circulation patterns tailored to each specific parking lot based on size, typical usage patterns, and proximity to destinations.
For major public lots drawing diverse users, I tend to favor a two-way flow for direct access. In small or low-turnover private lots, one-way simplicity may suffice. I also look holistically at orienting aisles diagonally to parking rows to naturally calm traffic and improve sightlines. Applying my expertise allows crafting optimized circulation plans enhancing user convenience.
Specify Pavement Design
One of the most important considerations in parking lot design is configuring a durable pavement structure that withstands heavy vehicle loads and environmental factors. With over 25 years of pavement engineering experience, I utilize American Association of State Highway and Transportation Officials (AASHTO) procedures to tailor designs to expected traffic and site conditions.
The AASHTO design process examines key inputs:
Subgrade Strength
This evaluates the load-bearing capacity of underlying native soils through geotechnical testing. Weak, compressible subgrades require thicker bases and pavements. Strong, stable subgrades allow thinner structures.
Traffic Volumes and Loads
The total number of vehicles, as well as the distribution of vehicle classes from small cars to heavy trucks, impacts stresses. Higher volumes and heavier vehicles require more robust pavement.
Climate Impacts
Temperature extremes, freezing cycles, and precipitation frequency dictate material specifications. For example, freeze-thaw durable aggregates are critical in cold climates.
Construction Materials
The availability and cost of quality aggregate and asphalt or concrete binders inform material selection. Long-lasting materials upfront save maintenance.
Given these inputs, I run AASHTO computations to determine appropriate pavement layer materials, thicknesses, and configurations to withstand projected loading for the design life. This prevents premature deterioration or failure.
Integrate Pedestrian Routes
Connecting parking conveniently and safely to building access points avoids conflicts through:
- Generous sidewalk dimensions (5-8 ft)
- High visibility crosswalks
- Traffic calming near pedestrian zones
- ADA-compliant grades, ramps, and surfaces
Plan Landscaping Elements
Trees, planters, and lighting fixtures enhance aesthetics while meeting functional needs:
- Shade tree placement
- Evergreen buffers along perimeters
- Low groundcovers and shrubs to maintain visibility
- Adequate fixture spacing for uniform illumination
Allow for Stormwater Drainage
Proper grading and inclusion of catch basins and piping prevent ponding:
- Minimum 2% cross slopes for surface runoff
- Area inlets placed to intercept sheet flow
- Detention basins to manage runoff volumes
- Water quality filtration as required
When designing parking garages, traffic engineers classify expected conditions using Level of Service (LOS) criteria to guide key layout decisions. Selecting appropriate LOS targets ensures the facility accommodates user needs and expectations. This article provides an overview of service-level considerations and design approaches.
Understanding Level of Service Factors
LOS classifies the degree of maneuverability, waiting times, and ease of use based on user familiarity as shown in this table:
Design Consideration | Acceptable LOS |
---|---|
Maneuverability | Employee → A |
Visitor → C | |
Travel Distance | Visitor → A |
Employee → C | |
Waiting Time | Visitor → C |
Employee → A |
For example, employees expect minimal waiting at entrances and easy maneuvering between their reserved spaces and exits. In contrast, visitors tolerate more circling to find a spot and longer exit lines.
LOS targets also depend on peak demand patterns. Facilities with constant high use warrant better service than those with short peaks. Urban settings allow lower LOS than rural sites where users expect immediacy.
Parking Garage Circulation Approaches
Four main circulation approaches, each with pros and cons, achieve different LOS targets:
1. Level Parking Bays
Parking occurs on flat floors with dedicated entrance and exit ramps.
Pros
- Excellent maneuverability for frequent users
- ADA-compliant accessible routes
- Adaptable layouts
Cons
- Additional ramps consume space
- Waiting at congested ramps
2. Level Drive Aisles
Sloped ramps connect level parking floors with no interleaving aisles.
Pros
- Simple, fast flow
- Compact ramp utilization
Cons
- Narrower spaces on sloped floors
- Limited accessible parking spots
3. Sloped Parking Bays
Each parking floor slopes down toward exits with level drive aisles between.
Pros
- Good utilization of ramps
- Wider parking spaces
Cons
- Indirect exit access
- Challenging pedestrian routes
4. Sloped Drive Aisles
Drive aisles slope down the facility with level, perpendicular parking on each side.
Pros
- Excellent utilization
- Faster exits
Cons
- Tight maneuvering
- No direct pedestrian exit routes
Optimizing Layouts for LOS Targets
When selecting a circulation approach, designers should consider:
- User Needs – Accommodate frequency of use and familiarity expectations
- Peak Volumes – Provide capacity for demand surges without excessive congestion
- Maneuverability – Meet needs of predominant vehicle types from compact cars to large SUVs
- Waiting Times – Limit queuing at exits by sizing appropriately and adjusting toll collection formats
- Wayfinding – Provide intuitive, clearly marked routing through the facility to destinations
- Future Flexibility – Allow for restriping adjustments if demand patterns evolve over time
With careful attention to balancing LOS targets across these factors, parking structures can be optimized for easy use, mimimal congestion delays, and long-term adaptability.
Conclusion
By thoughtfully applying AASHTO’s established parking lot design standards to the specific functional requirements, circulation patterns, vehicle types, and site characteristics of each facility, I optimize layouts to achieve the maximum parking capacity, safety, durability, and efficiency. My 25+ years of civil engineering experience enable expertly configuring parking assets to fully meet public needs while minimizing land consumption. Please reach out to discuss your upcoming parking lot design and site usage planning.
About the Author
Steve Axton is a licensed Professional Civil Engineer with over 25 years of experience planning and designing optimal parking lot layouts, traffic circulation, stormwater management, and pavement engineering. He serves as a parking consultant to architects, developers, and public agencies seeking to maximize parking capacity.
FAQs
How many accessible parking spaces does the ADA require?
ADA mandates accessible spaces in 5-10% of total spots depending on lot size, located closest to entries.
What is the minimum driveway width for 2-way car traffic?
For low-volume facilities, a 20-foot-wide driveway accommodates inbound and outbound vehicles.
What causes parking lot pavement to deteriorate prematurely?
Insufficient base preparation, poor drainage, excessive loads, and compromised materials lead to failed pavements.
Why is oil staining problematic in parking lots?
Petroleum dripping from engines prevents proper stormwater infiltration, impacting water quality.
What line striping standard should be specified?
High-durability water-based traffic paints with glass beads meet long-life expectations.
How can night visibility in parking lots be enhanced?
Strategic placement of bright, uniform security lighting improves comfort and safety.
What maintenance activities maximize parking lot longevity?
Crack sealing, seal coating, patching, and striping renewal preserve quality and function.
How does pavement thickness influence construction costs?
Thicker pavement requires more materials and time, directly increasing installed expenses.
What constitutes proper signage and wayfinding in lots?
Visible traffic control signs, space numbers, and directional arrows guide users.
How should landscaping be maintained around parking areas?
Regular mowing, pruning, and irrigation sustain health while meeting visibility needs.
Why should detention ponds be fenced?
Fencing improves safety by preventing access to pond facilities and avoiding accidents.
What are common LOS targets for public parking garages?
B or C during peaks. Less familiar visitors tolerate more inconvenience than daily commuters.
How can waiting times at exits be reduced?
Provide adequate toll booths and consider automated payment options to improve throughput.
What drives the need for larger turning radii?
Larger radii accommodate low-speed maneuvers of full-size trucks and SUVs.
How does parking angle impact aisle widths?
Right-angle spaces allow narrower aisles. Angled parking requires wider aisles for turns.
Why should designers allow for future restriping?
Restriping can adapt to evolving vehicle sizes and parking habits over the garage lifetime.
What factors determine the number of entrance and exit points needed?
Peak incoming and outgoing volumes, circulation patterns, and waiting time targets.
How does wayfinding signage improve the level of service?
Clear markings reduce confusion and improve user familiarity over repeat visits.
Why is ADA accessibility important in garage design?
Compliant routes must be provided for mobility-impaired users per codes.
How many accessible spaces should be provided and where?
ADA standards require a minimum number of spaces located closest to exits.
What contributes to poor maneuverability in garages?
Tight turning radii, curved ramps, cross slopes, and inadequate aisle clearances.
How can pedestrian safety be enhanced in mixed-use garages?
Separate marked walkways, low vehicle speeds, and good visibility.