Plantings can provide pedestrian and traffic control

Plants serve multiple purposes. They provide aesthetic beauty and human comfort. They produce oxygen and can help cleanse the air of particles. They provide food and shelter for wildlife and add to biological diversity. But most importantly, from a designer's practical-use standpoint, plants provide useful functions in the landscape. Planting for pedestrian and automobile control is an important function that you can fulfill with plants that guide, direct, slow and cool both people and their automobiles.

Desire lines If you want to use plants to persuade people to take a certain route, you will find the most success if you strategically plant around the walkway closest to the route people want to walk. The route people choose to take, which often is not a planned walkway, is called a desire line and looks like a cow path in a pasture (see Photo 1, at left, and Figure 1A, page C 10). If all of the facilities on a college campus were built without walkways, people would create their own paths of desired linkages between facilities. These desired lines are what landscape architects try to design when creating a pedestrian walkway system. They then add plants to these walks to make them more interesting, attractive and perhaps shaded from the sun, as well as to prevent pedestrians from straying from the path. However, the plants alone cannot control pedestrian traffic if the walkways do not follow desired lines.

Some people will walk through thorny thickets to take the route they desire (see Photo 2, at left). You must logically accommodate people's desire lines, or they will not use a planned path. When laying out walks, note the existing "cow paths" and observe the route people want to walk. Then you can begin using a combination of logical walkway layouts and plantings to create a practical and pleasing walkway.

Trees can enhance traffic safety Trees planted along streets and drives can increase traffic safety by slowing traffic to safe speeds and encouraging drivers to focus ahead instead of looking from side to side (see Photo 3, at left). Trees planted along roadways create a space or hallway for cars to move through. The trees become the walls and ceiling of the hallway when you plant them at regular intervals along each side of the road. By enclosing the roadway space, drivers slow down because the space is smaller than if no trees were there and the roadway space extended from building front to building front. This results in greater traffic safety and enhanced marketing of businesses. Drivers will get a good look at the few adjacent businesses instead of 20 businesses and 60 commercial signs at a time.

Street trees also forewarn drivers of upcoming curves. If the driver sees tree trunks curving ahead before seeing the road curve, he or she will slow down and be more cautious when approaching curves.

When laying out trees along streets, it is best to plant small trees such as red maples and 'Aristocrat' pears every 30 feet. You should plant larger trees such as sugar maples, oaks and cypresses about 40 feet apart for best results. By planting with this spacing, tree limbs will grow more upright rather than horizontal and downward, thus reducing the need to trim branches that have grown into the street space. To get a hallway effect where trees create a space for cars to travel through, it is best to plant them 6 to 15 feet from the edge of the road. The closer you plant trees to the road edge, the hallway effect will become narrower.

Pete Melby, ASLA, has taught planting design, landscape-architecture design and landscape-construction courses at Mississippi State University (Mississippi State, Miss.). In addition, he is a consultant, lecturer and author.

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