A major priority in civil site design is vehicle access and parking requirements. While these may seem like trivial aspects of an overall design, poor design decisions in these areas can mean unusable parking spaces, unreachable drive through windows, or inaccessible dumpsters. These are usually the types of mistakes no one notices until a site is constructed. Which means a designer may never be informed of the issue and realize he or she made a poor design decision, leaving even those with years of experience likely to repeat simple mistakes that make a big difference. Understanding how vehicles move is the first step to avoiding a poor design decision. This post focuses on design for general vehicle access that will help you sidestep any issues after construction.
Test Your Design at the Extreme Limits
Most site designers are familiar with the American Association of State Highway and Transportation Officials (AASHTO) guidelines and the various vehicle simulation programs built around them such as AutoTurn and Vehicle Tracking. These tools are essential for designers. These programs are mostly used when dealing with delivery trucks to ensure your retail site can be resupplied without much hassle, as we know truck access can make or break a retail site. But they can also be used to help identify where we can improve other site design areas like laying out parking. AASHTO guidelines are extremely conservative. So, while impractical to do all the time, it’s good to occasionally run a quick P Large Car model through the design just to get the feel of how it behaves. The P Large Car itself is 19 ft. long with a wheelbase of 11 ft, roughly the size of a 1975 Chevrolet Impala. If there are issues with your site, it should show by running a very large car through the design. There are hundreds of different types of cars on the road, all with different sizes and movement behaviors. Averaging out the worst aspects of most cars gives us the best reference for how well the site is designed. (Sidenote: You may find larger trucks and vans, but we often must consider other aspects like a ground clearance of 6” and a driver height of 3.5’ for sight distance, which are outside of the realm of truck or van).
3 Tips for Vehicle Access in Parking Areas
From drive aisles to bump-outs, here some general tips and guidelines for regular vehicle access in parking areas:
- Vehicles have a certain amount of swing. Though the front wheels do the steering, the rear wheels determine the turning radius as they are dragged through the turn. Putting smaller radii on the outside of a drive aisle may not look as nice as an even offset from the inside, but it is more practical in preventing cars from crossing into opposing lanes.
- Curbs are excellent at visually guiding traffic and protecting parked cars from potential accidents, but they have their own challenges. Hard, squared-off corners should never be used, even when delineating parking spaces. These “rim killers” don’t leave a good impression on your customers. I typically use a 3’ minimum radius on parking spaces and 6’ radius along lanes.
- Cars that pull into parking spaces must also be able to back out of them. On acute angled drive aisles, it’s easy to think you can park a row all the way to the corner. However, a car needs at least as much room to back out as the parking space is long. On dead end drives aisles, it may seem tempting to just do a straight edge from the side of a parking space across the drive aisle. Yet this leaves the driver with no room to maneuver. A bump-out in the aisle is necessary in this situation.
Navigating Drive Through Lanes
Another challenge for laying out commercial sites is drive through lanes. For program sites, it’s not so much of a problem because the future tenant will usually have their own set of standards to conform to. The trouble is when these standards are not set and one assumes that a drive through lane can be treated just like any other drive aisle. That’s a critical mistake. On a normal drive aisle, vehicles are expected to make use of the whole width, being able to use the opposing lane for their swing radius when there isn’t oncoming traffic. A drive through lane, with its service window, menu boards, and speaker box cannot be treated in the same manner. The customer is expected to hug the inside of that curb as close as possible, meaning the curb needs a larger radius to allow a smooth path. A typical minimum for this is 18 ft. That may sound huge, but it’s very reasonable as it prevents the natural swing of a large car from exceeding the typical 12’ width of the drive through lane. And because the lane is almost always occupied, you cannot rely on the extra space of a bypass lane, as a car waiting in line with their front end jutting out defeats the purpose of a bypass lane.
Every site is about compromise between what the client wants, what the jurisdiction requires, and what reality dictates. And when taking so much input into account, it’s easy to miss small design elements that make a big impact in the construction phase. Hopefully these simple tips I’ve picked up along the way will help you in your quest to design effective, efficient sites that work for your clients.
About Jason Mackey
Jason Mackey is a Designer for the Land Development – East Division in Peachtree Corners, Georgia and has been with Foresite Group since 2006. Jason’s focus is site design, utility design and grading. He has expertise is in AutoCAD, Civil 3D, and developing CAD standards and procedures.