Planning: Curb Lawns and Trees

A curb lawn, one name for a strip of vegetation running along a roadway, is a frequent element of rights-of-ways in America and other countries that have similar suburbs.  There is a wide variety of names for the feature, as documented here.  While often maligned for being nothing more than a monoculture of parched crabgrass, they can be important elements of streetscapes when utilized properly.

The best use of curb lawns is for planting trees.  Trees have a number of benefits to the users of a right of way, and to the residents that live nearby.  One is that they can provide shade for pedestrians, making their journey more pleasant on warmer days.  They can also shade hard surfaces such as sidewalks, roadways, and buildings, reducing the urban heat island effect.  Another benefit is that they absorb pollutants and particulates, improving the air nearby.  Trees benefit surrounding areas by absorbing storm runoff, reducing the chances of flash flooding, and the amount of water that has to be treated in combined sewer systems.

Trees also have less immediate benefits.  Foremost is that most people find a street planted with trees to be aesthetically pleasing.  This is generally reflected in property values, though it should be noted that may be more of a correlation than a causation.  Trees are pleasing not only for the shade they provide, but because they help shape the space for pedestrians, by providing a canopy that encloses and a prospect through which people can see other people and objects moving in the distance.  Large boulevards in European cities with double rows of trees excel at providing both qualities.

However, curb lawns have benefits even when they are not used to plant trees.  The primary benefit is to provide a buffer space between pedestrians and moving vehicles.  Without space to mitigate the sound and air movement created by a passing vehicle, many pedestrians find walking along busy roads unpleasant.  Another is that they allow for various streetscape items to be installed so they do not constrict or block the sidewalk.  Permanent items include signage, lampposts, telephone poles, fire hydrants, and traffic signals.  Other items may include bike racks, trash and recycling receptacles, restaurant seating, and benches.  These useful features add texture to the street and should be enabled as much as possible.

In narrower rights-of-way , where there isn't sufficient area for a continuous curb lawn, space for trees can be designed to alternate with space for parking.  Such arrangements still provide for both a buffer between pedestrians and moving vehicles, and most of the benefits of trees.  The compromise reduces the area for other vegetation and for the other minor uses that people find for the area, but are good nonetheless.  Busy roads with a line of bollards or similar devices to separate traffic from pedestrians may still be safe, but can appear unfriendly.

Some of the utility of curb lawns is dependent on climate.  In drier areas of the world, there may not be enough moisture for any vegetation to grow naturally in a curb lawn, let alone leafy, shade-providing trees.  Irrigation can be installed, but apart from high-traffic commercial areas, investing in such technology does not make sense.  It is reasonable in most arid regions to eliminate curb lawns along residential streets and place the sidewalk directly along the roadway, with the two areas only separated by a curb.  However, the important of a buffer space between pedestrians and vehicles should not be forgotten, and busier roads should still provide separation between the two groups of users.  The distance can be reduced with a physical barrier of sufficient height, but those are expensive and are mostly used along limited-access highways.

Ultimately, the presence of trees and curb lawns is a cultural practice.  There are many successful urban environments where neither are present.  However, there are good reasons for them to be installed where rainfall is sufficient.  They should not be discarded in the pursuit of either maximum density or maximum parking.