Planting street trees for increased road safety
The school of thought in designing roads and any associated vegetation, has been to favour a design philosophy of “forgiving” roadsides that provide wide shoulders and clear, tree-free offsets. That is, driver mistakes happen and safe roads and streets are those that forgive mistakes.
Increasingly though, there is an appreciation of the numerous and important benefits of street trees such as reducing heat, improving walkability, increasing property values, beautification and catalysing property development, as well as a swag of environmental benefits etc.
These days, in the planning and design for liveable cities, road engineers advocate for roadside vegetation and trees that are placed appropriately, based on actual crash risk rather than generalized assumptions. Indeed, road designers realise that high speed road and highway standards are not appropriate for lower speed streets.
In fact, trees can make streets safer, and some studies have found that for suburban roads,
the presence of trees significantly dropped the cruising speed of drivers by an average of 4.87 kilometres per hour.
faster drivers and slower drivers both drove slower with the presence of trees.
trees on urban roadsides may be associated with reduced crash rates.
people perceived streets with trees as the safest streets and urban streets without trees as the least safe streets
some roads that have had streetscape enhancements have seen reductions in the severity and frequency of crashes.
The following graph shows some of the results comparing speed in streets with and without trees. Now it seems that the authors have been to the same school as me in terms of making a small difference look bigger by starting the Y-axis closer to the lowest scoring category, but hey, a 7% reduction in speed is still a good result in terms of safety.
Given the safety risks can be reduced or increased with street trees, the following from NSW seems a reasonable site specific risk-based response:
1. 50km/h and below streets : there are no minimum street tree setbacks required
2. 60km/h : follow a context sensitive, risk-based approach to the provision of trees. Tree setbacks less than the standard dimensions (around 2–3m) may be justified when combined with consistent and current road geometric standards as well as whether kerbs or barriers are used.
3. +70km/h: Tree setbacks should be based on the appropriate clear zone dimensions for that speed.
Full credit to the many engineers and planners that not only really value the numerous benefits street trees can bring but also see them as tools to increase the safety of our urban environments.
There are lots of good guidelines and information on this …but here are a few worth a read.
General design direction for streets and roads: NSW government
The street tree effect and driver safety. ITE Journal on the Web. Naderi, Jody & Kweon, Byoung-Suk & Maghelal, Praveen. (2008).
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