All you ever wanted to know about curb-bumpouts, and never knew to ask

City streets are designed to accommodate many functions.  The slope of a street and the height of a curb help direct stormwater to the drain, while the width of the street and the presence of street trees send important signals to drivers whether to speed up or to slow down.  The geometry of the sidewalk is also important, and one of the most popular features in street design today is the curb bumpout.

A curb bumpout is essentially an extension of the sidewalk into the street.  They can be used to shorten crossing distances on streets, thus making it easier for pedestrians (particularly older pedestrians) to cross the street safely.  They are also increasingly being used as opportunities to insert small landscaped spaces that also act as stormwater catchment basins, which help reduce stormwater overflow on city streets. Curb-bumpouts, like the streets they line, serve many purposes.

However, as curb-bumpouts increase in popularity, particularly among urban design connoisseurs (which everybody seems to be, these days) it is important that people recognize the many design and management problems associated with them.  When you add a bumpout to a city street you actually have to change the official City Plan. In Philadelphia long bumpouts require an ordinance to be introduced into City Council while smaller bumpouts require only a letter from the Streets Commissioner and a public hearing.

Curb-bumpouts not only affect how we move, but they have impacts on the utilities below our streets and the neighborhoods above.  You don’t want to put bump-outs over parts of the street with utilities running through them because it makes it harder for utilities to make repairs to their pipes, wires and infrastructure.  Bumpouts should not be placed where they negatively impact the turning movements of fire-truck or buses: though sometimes they can be designed to accommodate those movements.  On many narrow streets constructing bumpouts may not be worth the time and money to construct them.

Bumpouts have gotten a lot of press because of their ability to absorb stormwater.  Communities across Philadelphia are excited by Stormwater Bumpouts because they add green space on residential city streets.  However it is important to realize that stormwater bumpouts can’t be placed just anywhere, because the most important part of these specific bumpouts is not what is above ground, but what is below.  Stormwater bumpouts (32 of which are currently planned for Philly) are designed to absorb the first inch of stormwater for 1 acre of right of way.  That is, they are designed to absorb the water that runs along 1 acre of streets and sidewalks.  They can only be placed where inlets already exist, because their job is to capture that rain before it enters the inlet and slowly release the water into the system after a storm.  Often their storage tanks extend 100 feet up the street beyond the stormwater bumpout.  Just as important as their physical design, is the presence of a strong community group that can take the responsibility to keep the stormwater bumpout clean, picking out the trash that people may throw into the vegetation.

Bumpouts are not just about making the streets look better, they are for improving safety and managing stormwater.

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