NB: I believe this topic has been discussed much better many times elsewhere on the net and in print. I am writing simply to clarify my own thoughts. I apologize for the primitive graphics.
One of the issues that arises with higher-density housing is parking. As I discussed in a previous post, people want cars and thus parking for a variety of reasons. On-street parking in residential areas has a number of issues, and off-street parking does as well. Nonetheless, people desire parking, and there are ways to accommodate it in the context of fee-simple houses - up to a certain point.
The standard way to accommodate parking is to build access from the street to the private property where an owner wants to park vehicles. In low-density suburbs and rural areas, that means a simple paved or unpaved driveway leading into the property from the roadway. As housing becomes denser, in most cases the access crosses the pedestrian portion of the right-of-way, which is usually a paved sidewalk, but may in some cases be bicycle lanes. These intersecting paths of movement are point of conflict between vehicles and pedestrians. They are also known as curb cuts in America, because the standard high curb is cut down so the vehicle can drive smoothly from the street into a private property.
On lots with large, open yards, it can be easy for a driver to spot a pedestrian. (In this post, I will use the word 'pedestrian' as a shorthand for any using the sidewalk. Apart from people walking, other users may include joggers, younger cyclists, micromobility users, assisted mobility users, and older cyclists, depending on the local regulations and specific configuration of the right-of-way.) On lots where vision is obstructed by plants, fences, or other buildings, pedestrians can be injured if a driver is not careful. Pedestrians learn to be wary in low-visibility situations, but too many dangerous crossings can make walking down certain streets unpleasant, and those streets are avoided.
Another issue is that curbside portion of a street where the access to private property ends becomes an area that can't be used for parking if there is on-street parking on the same side of the street. In that situation, it can't be used for normal vehicular movement, either. In some neighborhoods, an informal rule develops that a property owner can block the access to their own property, but doing so still risks a ticket. It can also encourage less conscientious drivers to block other people's driveways.
A third issue with curb cuts is that sometimes people simply park their vehicles across the sidewalk, forcing pedestrians to detour and potentially enter the roadway to continue their journey. This happens most often when the driveway is relatively short and a larger vehicle such as a pickup does not fit completely onto the private lot. It also happens when the driveway is obviously too short, but the vehicle fits between the end of the driveway (usually a garage door) and the street. Either way, in most jurisdictions this discourtesy is illegal, but rarely enforced unless someone complains.
One solution to the issue of curb cuts is to create a second, usually parallel, right-of-way that allows rear access to properties for the purpose of parking. This right-of-way is usually called an alley, but is sometimes called a service lane, or just a lane. Alleys predate the automobile by hundreds or thousands of years, depending on how they are defined. Some American cities that have them were laid out when the horse was the main means of long-distance travel. But alleys have been adapted to support motor vehicles, and offer some advantages over front access for parking.
The first is that alleys cut down on the opportunities for pedestrian-vehicle conflict. Alleys reduce the number of curb cuts from many to as few as one per block, though two is most common. While pedestrians can use the alley as a walkway, most people prefer a sidewalk, as an alley is an obvious zone of conflict. Some urbanists are interested in upgrading the alley to a sort of second, pedestrian friendly street, but outside of warm tourist towns, that seems to me to be a duplicative and futile effort.
The second advantage is that with rear parking and reduction of curb cuts, the street in front of homes has more space either for parking or the various forms of traffic, depending on how it is striped. Or the roadway can be narrowed and restricted to vehicle movement only, reducing the amount of paved area the jurisdiction needs to maintain. The space freed in this way could then be used to create or expand curb lawns.
Alleys have some smaller benefits. They are good places for trash pickup, as a garbage or recycling truck can access cans or bins in an alley without having to pull them across a line of parked cars from the sidewalk or curb lawn. They also reduce the chances of the truck driver coming into conflict with moving vehicles, as alleys are usually so narrow that most passenger vehicles cannot pass a stopped truck. They can also be used for overhead utilities in areas where they cannot be placed underground due to costs or technical reasons. Overhead wires in an alley don't mar the appearance of the adjacent street, and they don't come into conflict with any trees planted in front of the homes.
Alleys have downsides, of course. The primary one is that they have to be built and maintained. While they don't have many of the features of streets, such as sidewalks, curbs, crosswalks, lane markings, and traffic lights, they still need to be built to support the vehicles that travel down it. Garbage trucks are the heaviest vehicles to travel through residential areas on a regular basis, and cause the most damage to residential streets from vehicles. The second is that the hard surface of an alley will increase runoff over that of unimproved yards. The tradeoff between having an alley and rear parking verses having driveways and front-parking needs to be calculated when it comes to managing storm water.
Another negative negative factor is that alleys tend to be unsightly. Because they may not be fully visible from the surrounding buildings, trash can be dumped and improvements vandalized without anyone even being able to notice. And the fences and structures along alleys are often neglected since they aren't the public face of the property. Alleys are problematic in areas with heavy snowfall. Because there is nowhere to put the snow, they are left unplowed, making access difficult or dangerous. Snow can be moved with a bucket-loader and trucked out, but that is an expensive option.
Alleys also have a reputational issue. The dark alley is a widespread fictional device that probably has its roots in the actual alleys of medieval cities in England. There is no doubt that the originals were poorly lit and often the location of crimes that might not take place in wider streets or the daylight. Modern alleys are not narrow walkways with sections passing beneath buildings, and often have streetlights. Nonetheless, the negative perception continues, probably because alleys tend to be trash-strewn and poorly maintained, indicating that few people notice or care what happens in them. Research indicates that gating alleys in the UK reduces crime by a small amount, but the number of studies is limited. The overall level of crime in an area with alleys is probably more important than the absence or presence of alleys themselves.
Because of their complications, I think alleys need to be analyzed with caution in mind before they are included in a design. I think they provide a way to mitigate the issues of parking in residential areas of moderate density, but they are not a solution in themselves. Only by addressing car-dependent mobility can the problem of parking be truly solved.
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