Roadside Trees the Smart Way
If you’re a designer creating a new streetscape plan, a civil engineer putting the finishing touches on your road project, an environmental consultant preparing a mitigation proposal, or a community improvement district landscape architect preparing a showplace entrance for your local interchange, chances are you will be using trees as the primary vertical design elements of your planting scheme.
You may be thinking that anything you draw would be an improvement. Your proposal may not be as benign as you hoped, especially when trees are involved. Trees are a statistically significant cause of traffic deaths and a frequent trigger for utility outages. They can pose a big problem for maintenance crews, and they can block sight lines for pedestrians and cars trying to enter or cross the roadway. Businesses and sign owners can be very upset if your proposed trees block their signs.
People plant trees all the time, because trees provide wonderful shade, color, wildlife habitat, buffers, and visual beauty to the landscape. When located wisely, trees can provide all the benefits, without any of the worries. You can avoid a lot of problems by locating your new trees well beyond the clear zone.
The clear zone is typically the width of the roadside beyond the travel way where an errant vehicle might possibly recover and get back on the road. If it is kept clear of fixed objects, then most of the vehicles that run off the road can get back on the road safely. If there is a fixed object the area that should be clear of fixed objects, then the driver is not so lucky. Curbs do not slow vehicles moving over 25 MPH, and vehicles that hit fixed object going over 35 MPH usually result in considerable injuries or death.
The clear zone is based on the horizontal curvature of the road, traffic volume, the speed limit of passing cars, and the slope of the adjacent roadside. The faster cars go, and the steeper the downslope, the wider the clear zone area will be. If you can’t “meet the clear zone” minimum setbacks, then don’t plant the tree, with notable exceptions.
Street trees get a reprieve from the calculated clear zone requirements, because they share the roadside space with utility poles and bollards and street furniture, and because drivers are more alert to pedestrian activity and surprises in urban settings. In central business districts with lots of pedestrians and established landscape strips, local and state governments may adjust their required setbacks to balance the safe offsets with the benefits and aesthetics the tree canopy and tree form provide. There is a limit to this generosity, and state regulations may override and be more restrictive than local ordinances. The limit is, naturally, the amount of space a tree needs to survive without damaging the nearby curb and sidewalk areas. If you press for a variance beyond these already liberal tradeoffs, then you will lose your trees within just a few years and damage the costly hardscape elements as well.
Keep Pine trees at least 60 feet away from the edge of the road, because they will fall as they age, and will drop limbs on a regular basis. Their wood is simply not strong enough to be reliably safe around vehicles. Keep sight lines for drivers and pedestrians entering the road clear of trees. The trunks may be insignificant at first, but large street trees have trunks that can quickly grow to three feet in diameter or more. Locate trees at least 10 feet away from any underground utilities and at least 20 feet away from the outside phase of any power line, for obvious reasons. If you are determined to plant under power lines, then keep your choices to trees that mature to less than 20 feet. Trees that are topped and deformed by utility crews are just plain sad. A little forethought on your part can keep them safe from the chopper and the chipper.
Place trees in areas that will be mowed in consolidated beds, where they are a maximum of 10 feet on center. That way their own leaf litter will self-mulch the beds, and mowers can easily drive equipment around the beds rather than in tiny circles around each tree. Any tree you propose in turf areas that are not in consolidated, mulched beds should be deliberately placed for a special design intent, because each one of these individual trees requires its own individual mulch ring, and mowers must circle around every one of the mulch dots within the turf area. Be considerate of the guys on the mowers, and don’t force them into circumnavigating hundreds of trees several times each mowing season and providing new mulch for the rings four or five times each year. They might get dizzy and drive out into traffic!
Billboard owners are increasingly political and vocal about visibility of their signs adjacent to public rights of way, and store owners have major investments in locating in downtown areas of commerce. Think about the view from the signs down to the drivers, and keep it open. Check with restrictions on planting in front of businesses and signs. You may be surprised to find your plans rejected if they block visibility within a typical 500-foot view zone for a billboard.
Remember, too, even little bare root seedlings used for reforestation will grow to be big trees, so treat them as if they are at least 40 feet wide and 60 feet tall as you design them into your mitigation plans.
By knowing early these typical restrictions for roadside design, you can set up clear tree-planting zones within your landscape design. Once in place, the design direction of your project becomes straightforward, without difficult permitting or project review delays.
You can find more information about trees in The Advanced Guide to Trees: Choose Trees like a Pro, part of The Advanced Guide series.