Macdonald on Street Trees and Intersection Safety

Street trees: valuable neighborhood amenity or safety menace? Many traffic engineers and municipalities take a dim view of arboreal canopies near corners. Now, offering a more positive view, comes Street Trees and Intersection Safety by Elizabeth Macdonald et al. It is probably the most complete investigation of this topic to date, and is nicely readable (for an academic study, that is). Here is some of the opening paragraph:

The study derives from a rather simple, straightforward observation: that on the best tree-lined streets the trees come close to the corners. They do not stop at some distance back from the intersecting street right-of-way. Indeed, in Paris, a city noted for its street trees, if the regular spacing of trees along the street runs short at an intersection, there is likely to be an extra tree placed at the corner. For at least 250 years, the finest of streets the world over have been associated with trees. (p. 7)

The study looks at regulations that have the effect of prohibiting trees on corners, or of stripping street corners bare. Often these regulations are ad hoc and discretionary:

In many cases, arborists, engineers, and code enforcement officers told the researchers that a particular standard was not available in writing — it existed only as a rule of thumb, an informal agreement between departments, or a series of professional judgments made on an individual basis. …The widespread use of discretion means that urban designers are likely not to know the rules of the game or to be able to successfully challenge them. (pp. 13, 82)

While tree regulations are highly restrictive, ordinances controlling similar visual blockages such as parked cars and newspaper racks are much less so (particularly in the California cities that were studied). Newspaper racks are often unregulated; where regulations exist they are minimal:

Although setbacks from intersections are usually required, they are much smaller than for street trees, even though newspaper racks are much larger and bulkier. Where marked crosswalks are provided, the required setback is often as little as 3 feet. (p. 52)

The net effect of regulations in many cities is that it’s difficult or impossible to build streets that are as pleasant and leafy as the best existing streets.

A few arborists stated explicitly that the standards make it impossible to replicate their city’s most delightful streets and neighborhoods. … Indeed, given setback standards for trees of 50 feet or more from intersections and given short blocks, the result can be so few trees along a street as to be meaningless in terms of any positive impact. On a typical Portland, Oregon, 200-foot wide block, for example, such a setback standard combined with a not uncommon 50-foot spacing standard would result in just three trees per block. (pp. 56, 9)

The experimental section of the study involved video simulations of different street corners. One was configured according to AASHTO standards, one used Oakland, CA’s standards, and one was an “Urban Preferred” configuration with street trees near the corner and on-street parking set back from the corner. The Urban Preferred scenario performed the best.

The simulations tested suggest that in urban situations street trees near intersections cause less of a visibility problem than either cars parked in on-street parking spaces or newspaper racks. Put simply, a strong case can be made that street trees planted close to intersections and reasonably closely spaced, as little as 25 feet apart, which are pruned so that horizontal limbs and leafing start about 14 feet off the ground, do not constitute a visibility safety hazard on urban streets. (p. 83)

Finally someone is recognizing the safety hazard posed by SUVs that are parked. SUVs block sight lines like trucks, but unlike trucks they are allowed to park in locations where they become hazardous.

Rather, on-street parked cars, particularly large ones such as SUVs, create substantially more of a visibility problem. … Although it might be hard to put into practice, and to police, cities might also consider restricting tall or bulky vehicles from on-street parking spaces near intersections — something of a “compact car only” zone. (pp. 81, 83)

So if you want to geek out on vehicle and street tree interactions, give this study a read.

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