How car positioning affects traffic safety
Our transportation infrastructure depends on available parking. Large destinations offer surface lots, streets in front of stores provide parking at the curb, and downtowns are dotted with multi-story parking facilities. The parking stall is so ubiquitous as to be unremarkable, except when the driver is in a hurry and cannot find an open place. Yet parking design should be well-researched to achieve the aims of a community. What are the advantages of street parking vs. lot parking? How do passenger vehicle needs differ from those of commercial vehicles? What style of parking best serves passenger and vehicle safety?
What are the advantages and challenges of street parking?
- Great for high-turnover and quick-trip purchases. People don’t park in a garage to grab a take-out coffee or sandwich, as it would require parking, coming to grade, paying, walking to the store, and then reversing the process.
- Cars provide a steel buffer to pedestrian (and sometimes cyclist) spaces.
- Street parking can offer more efficient use of land than lots in high-value urban areas by only offering infill space to vehicles, rather than taking a space which could hold a building.
- In urban cores, limiting parking by offering mostly at-curb parking encourages other modes of transport. People may choose to bus, walk, or cycle in. This discourages traffic snarls.
- A high turnover of vehicles, encouraged by short-stay parking, can add vitality to an area.
- Metered street parking often goes to the city rather than to a private company.
- Street parking, especially on two-lane streets, can have the net effect of slowing traffic down as people must wait for drivers to reverse into or out of a space. In pedestrian and high-volume areas, this slowing of the flow of traffic can lower the frequency or severity of accidents.
Street parking orientation
Street parking can be offered in several orientations: parallel to curb, drive-in angle parking, and very rarely, back-in angle parking. A study in Australia on these parking variations highlights a few interesting facts.
Regardless of the style of parking, reversing takes more time than driving forward. In each of these forms of parking, a vehicle must reverse. To parallel park between two cars, the reverse happens when a person is entering a spot, backing in. This is also true of back-in angle parking. Drive-in angle parking places this maneuver requiring extra time on leaving a spot: to pull out of the stall, the driver must back into the traveling lane.
The study also suggests that claiming a parking space slows traffic more than leaving a parking space does. When drivers are pulling into traffic, they will usually wait for a gap. In contrast, someone parking will do what they must to maneuver to an open spot. This suggests a slight advantage to drive-in angle parking, as the car is in reverse only on leaving a parking space, when the driver will be trying to leave during a break in the traffic. However, it’s hard for an angle parked driver to see over the backs of other cars. This makes it difficult, and potentially hazardous, for drivers to leave their spot.
Advantages and challenges with parallel parking:
- Drivers can often easily spot open spaces
- A parking lane provides a buffer between pedestrian spaces and traffic
- Entering a parallel parking space often takes more time and skill than entering drive-in angle parking
- Requires 7–8 feet of space curb-to-traffic in parking lane
- Parallel parking is quicker to exit than drive-in angle parking
- Car doors can be a threat to bikes or bike lanes
- Stowing packages in the trunk happens between cars rather than close to traffic
- Drivers can be unsure of the maneuver: one study found three in ten drivers are so afraid of parallel parking they will drive out of their way to find a space that does not require reversing the car
Advantages and challenges with drive-in angle parking on the street:
- Nosing-in is the quickest form of parking
- Exiting requires reversal, which takes longer, but drivers try to do this during a break in the traffic
- Angled cars provide a large buffer between pedestrian spaces and traffic
- Opening car doors are not a threat to cyclists
- Requires up to 19 feet of space curb-to-traffic in parking lane
- More cars can be parked per block
- Room for angled barriers with space for plants or street furniture
- Traffic must go even more slowly due to surprise unparking than with parallel parking
- Stowing packages in the trunk happens near traffic
- Cars are “aimed” at sidewalk and stores: rolling, collision, or gas/brake pedal confusion are more likely to crash cars into shopfronts unless there are bollards installed
- Reversing into traffic can be quite dangerous since drivers have trouble seeing over the backs of the cars beside them, especially with size mismatch (trucks and SUVs beside compact cars.)
- Large vehicles can obstruct view of empty spaces
Advantages and challenges with back-in angle parking on the street:
- Driving out is very simple with back in angle parking, working like a merge, and can be quickly done even in busy traffic
- Visibility upon exit is not the same issue as drive-in parking
- Ease of access to trunk allows shopping to be stowed away from traffic
- Opening car doors are not a threat to cyclists
- Doors opening create barrier between passengers and traffic, guide children toward sidewalk
- Creates a large buffer between vehicles and traffic
- Requires up to 19 feet of space curb-to-traffic in parking lane
- Large vehicles obstruct view of empty spaces
- Backing in on a 45° angle is a less complicated maneuver than parallel parking
- Conversely, very few drivers have practiced reversing at an angle. Many are hesitant or even put off, and therefore the time getting into these stalls can be very high or people can end up parking through many stalls at a variety of angles
- Cars have a hard time judging the distance to the curb unless bollards placed as visual guide to the end of the stall
Surface parking—lots and multi-story garages
Stacked in garages, or spread around a building like a moat, available parking draws traffic. Although there are some destinations that can replace parking with transit and great cyclist facilities, many people are car dependent, and whether a driver can find a place to leave a car at a reasonable distance from a destination can make a difference between choosing one place over another. Until infrastructure changes, some parking lots will continue to bring high value to the public and cannot be easily disrupted by other modes of transit. A shopping destination with voluminous average purchases (i.e. Costco) is more dependent on parking than a market that offers smaller ranges and sizes of goods. Hospitals and other emergency areas also rely on parking to provide services to patients and families who travel all manner of distances in all sorts of conditions.
What are the advantages and challenges of parking lots?
- Usually well lit, often patrolled or observed by many people
- Bring many people in proximity to a resource
- Disability parking can give otherwise mobility challenged people access
- Multi-story car parks or garages can create revenue and provide a density of parking
- Large paved areas are not permeable to water, and can exacerbate flooding and create water pollution
- Building multi-story lots in busy urban areas sometimes means tearing down existing buildings and can create backlash
- In suburban and rural areas surface parking often adds to sprawl and “parking deserts”
- Large, dark paved surfaces create urban heat islands
Parking lot orientation
As with street parking, parking lots can be oriented in a few ways. Angle-in parking and parking perpendicular to traffic flow are the possible car orientations in parking lots and structures, but choosing which makes best sense has a lot to do with available space, user requirements, and likely volume of traffic.
In most parking lots, spaces are square to traffic lanes. People can either nose-in to the stall or drive past the stall and reverse into the space. This backing-in skill provides the same benefits of back-in parking on streets. When leaving the space, it allows the exiting driver to see all traffic, without the ends of other vehicles obscuring their field of vision. For this reason, back-in parking cuts down on accidents with other cars or pedestrians. Still, backing into a space is enough of a challenge for some drivers that there is an entire website dedicated to the skill displayed in parking lots, called “Fancy Parking.”
Features of 90° or perpendicular parking
- Most parking lots offer these stalls, with two-way lanes between them
- Using two-way lanes allows for short or dead-ended aisles, which allows the designer to fit spaces into all sorts of lot shapes
- Usually installed allowing two-way movement of cars and two travel lanes
- Two travel lanes give extra room for maneuvers, which can be helpful in decreasing conflict between patrons
- Two travel lanes mean drivers can leave in the appropriate direction
Features of angle parking, 45° and up
- Very easy for people to nose-in park
- Back-in angle parking a challenge for many drivers but makes backing out very simple
- Easy trunk access with nose-in parking, slightly more “tucked in” out of traffic than 90° stalls
- Depending on the site, different angles and stall patterns can be used to fit cars, very helpful for narrow sites
- Angle parking allows one-way traffic lanes which can be much narrower
- One-way traffic can be confusing and ignored, leading to conflict
- Some people may try to back-in in a nose-in parking lot, or vice versa, leading to their car being pointed against traffic
- Signs and surface painting must make desired traffic flow very clear
- Drivers must wait for those backing out in front of them, preventing passing: this can slow down exit, but can also avoid the conflict or gridlock caused by passing in busy 90° lots
- There are triangles without cars at the end of every aisle: this reduces number of cars on the lot but can be used to help with storm water processing
In parking lots, vehicle stops can be used at the end of stalls to prevent people from taking two spots or driving through spaces rather than moving in travel lanes. Long low stops are often optimally placed on the periphery rather than the interior of the lot, or along pedestrian corridors. In the middle of the lot they can make cleaning or snow removal more challenging, and they are a tripping hazard to the unwary user. Rugged flexible poles can modify some of these issues if used mid-lot, to guide traffic and demark spaces without taking up as much area on the ground.
Parking and infrastructure design
The ubiquity of parking lots and street parking can make the decisions about them almost invisible to the average driver, yet the decisions about the orientation of cars, one- or two-way flows of traffic, and bollards, stops, and curbs around the parking are all considerations for the safety of drivers, cyclists, and pedestrians. Parking lots are often a first test of engineering and planning students, managing waste water, urban heat islands, and optimizing safety for all users. Will cars be oriented in such a way as to keep opening doors away from cyclists, and debarking children away from the street? Will backing-up maneuvers cause traffic confusion or chaos?
As driver-assist features and autonomous cars evolve, they will likely change the face of parking. Back-up angle parking offers the most safety to cyclists, passengers, and drivers, and is mostly a challenge due to driver inexperience and tension during backing up. If driver assist features become popular, this type of parking may become more popular. Yet in a future where autonomous vehicles become the norm, parking may become substantially less important. Driverless vehicles pick us up and drop us off as needed, rather than waiting, in a concrete desert, for us to return to them.