Where are Bump-Outs Most Effective? Where are Bump-Outs Excessive for Development?

Bump-outs (also known as “curb extensions“) have become commonplace in many subdivisions across the country.  They are also common in the existing neighborhoods as a means of traffic-calming.  The purpose is to provide an additional element in protecting the vehicles parked on the street and enabling shorter, safer crossing for the pedestrian at the intersection.  The bump-out can be a costly add-on for a neighborhood considering the additional curb that is involved and additional measures that need to be taken to handle stormwater.  Careful consideration should be given whether a bump-out is most appropriate per the situation.  In the instances where bump-outs are desired, I have developed a few variations of the bump-out that can effectively manage stormwater with infiltration, calm traffic and ease the costs of additional curbing.

In this post about bump-outs, I will talk about the instances along the urban-to-rural transect where the bump-out is excessive and other design alternatives can be equally, if not more, effective.  For all neighborhoods, districts and areas along the transect that are T-3 (Sub-Urban) to T-1 (Rural Preserve), the bump-out is unnecessary.  Vehicular trips per day in these areas are low so the design of the road cross-section should be narrow enough that vehicular speeds are decreased throughout, not just along at the intersections.  Traffic design speeds in these areas should be no greater than 25 mph, yet the excessively wide streets typically provided lend themselves more for airplane landing and take-off speeds.  The typical automobile driver does not even see the majority of street speed limit signs.  The typical driver will use environmental cues to determine the speeds that should be traveled, the most fundamental cue is the width of the street and how much space is dedicated to their automobile.  On a conventional suburban street, which typically measures a minimum of 36-feet curb-to-curb, there is dedicated space for on-street parking.  However suburban streets rarely have vehicles parked on the street.  Most vehicles are parked in garages or in the ample driveway space created by Euclidian zoning setbacks.
A narrow cross-section is more often more appropriate for traffic calming than bump-outs.

The graphic to the right illustrates a narrow street cross-section for two-way traffic measuring 20-feet to 22-feet in width from curb to curb.  Adjacent to the rolled curbs (gradually sloped curbs that permit vehicles mounting the curb) are 7-feet of porous, or pervious, surfaces to accommodate parked vehicles.  This strategy creates a safe environment for the pedestrian, or kids playing, and a generous buffer between the sidewalk and vehicular traffic.  Cost savings for the developer in street widths allow for greater amenities within a neighborhood, which could include additional parks, more street trees, enhanced landscape among many other opportunities.

This image, by Maggie Osterberg, is an example of the planter that is not only a beautiful addition, but also very effective in decreasing pedestrian-automobile conflicts.

Bump-outs can also be eliminated in very urban areas (T-4 to T-6) and be better managed with narrower streets or other elements that can effectively create the bump-out.  A beautiful and eclectic example of alternatives for bump-outs is in the Historic Old Market District of Omaha, Nebraska.  In the Old Market, raised planters act as a bump-out tightening the streets at the intersection, as shown in the image to the right.

South Main, in Buena Vista, utilizes this technique without the rolled curb and with vegetated surfaces to accommodate the vehicle.  In the instance of South Main, generous space is provided for parallel parking with street trees spaced approximately 25-feet on-center.  (See Buzz Boulevard: A Context-Sensitive Model for Neighborhood Boulevards for a photo and more information)
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