The door zone is the space spanning about four feet (1.2m) from the sides of parallel parked cars. It is hazardous to ride a bicycle or motorcycle in a door zone because if a door suddenly opens, the cyclist must either crash into it or swerve into the adjacent lane of traffic. An opening door may also knock the cyclist into passing traffic, leading to serious injuries or death.  Regardless of this deadly hazard, bicycle lanes or cycle lanes are often painted within door zones.
Most areas have laws that require car users to check for bicyclists before opening the door of their vehicle, but there still have been countless injuries, including numerous deaths, caused by cyclists riding in door zones when a car door is carelessly opened. The problem is that cars doors are opened carelessly all the time, without any ramifications, because usually nobody is traveling within the door zone, and such laws are practically impossible to enforce, so it's easy for people to fall into the habit of opening doors without checking first.
Although many areas have laws that apparently require cyclists to ride on the outside of the road, these laws have many exceptions, and compromising safety is not legally required. 
Because it is often not possible to see and react safely to a suddenly opening door, techniques such as effective cycling, and vehicular cycling, teach that cyclists should ride outside of the door zone of parallel-parked vehicles along the roadway, which generally means riding at least five feet away from the edge of parked cars. When riding in this position invites motorists to squeeze into the lane too close to the cyclist, traffic cycling experts recommend using the full lane to avoid sharing the lane side by side with other vehicles.
- ↑ "Of the eleven cyclists who were killed [by dooring], nine of them were actually killed by another vehicle passing by. One of the nine swerved to avoid getting doored, lost control of his bike, fell, and was struck by a passing eighteen-wheeler. The other eight were knocked into the path of another vehicle after first striking the door. The other two cyclists who were killed died after striking the door and being thrown to the pavement." Bob Mionske, Bicycling & The Law, p. 153
- ↑ With regard to riding close to other vehicles, The California Motorcycle Handbook states: "Riding between rows of stopped or moving cars in the same lane can leave you vulnerable. A car could turn suddenly or change lanes, a door could open, or a hand could come out of a window."
- ↑ "No person shall open the door of a vehicle on the side available to moving traffic unless it is reasonably safe to do so and can be done without interfering with the movement of such traffic..." CA CVC 22517
- ↑ "Thus ... you are not required to choose between safe riding and obeying the law". Bob Mionske, Bicycling and the Law, p.155
- ↑ "At 15 mph it takes more than two car lengths to recognize a danger and stop, and you can't see the danger two car lengths ahead. If someone opens a door close ahead of you, you have only one choice: dodge out into the traffic lane. It is much safer to ride there consistently in the first place. So ride far enough away from a string of parked cars to clear an open door. If there are gaps in the string of parked cars, don't dodge out of the traffic lane between the cars. Because entering a traffic stream is one of the significant causes of car-bike collisions, don't make yourself enter the stream more often than necessary." John Forester, Effective Cycling, sixth edition, pp 296-297
- ↑ "When passing parked cars, stay toward the left portion of your lane. This way, you can avoid problems caused by doors opening, drivers getting out of cars, or people stepping from between cars. If oncoming traffic is present, it is usually best to remain in the center portion of the lane to maximize your space cushion." California Motorcycle Handbook
- ↑ "Bicyclists should ride far enough away from parked vehicles to avoid being hit by an opening door." California Department of Motor Vehicles Safety Tips for Bicyclists and Motorists (FFDL-37)
- ↑ "Ride far enough to the left that you won't run into any door that's opened unexpectedly. Michael Bluejay, bicyclesafe.com
- ↑ "Luckily, most streets are sufficiently wide that a cyclist can ride between the DZ and the path of passing traffic at a constant, comfortable pace. It is only on the really bad streets that the cyclist is confronted with the necessity of commandeering a busy traffic lane to avoid the Door Zone..." Robert Hurst, The Art of Urban Cycling, p. 113