Walkable West Palm Beach

How to make a great Southern Boulevard Bridge – Part #2

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Since our last post the WalkableWPB team has been busy researching the proposed design of the Southern Boulevard Bridge and we have crafted an alternate design that would provide proper bike and pedestrian facilities with a $1.15 million dollar cost savings from the current FDOT proposal, shown in the following rendering:

Southern Boulevard Bridge - FDOT

Southern Boulevard Bridge – FDOT

The current design is very aesthetically pleasing and even pedestrians have been taken into consideration by providing a pergola on the bridge as shown here:

Southern Boulevard Bridge - FDOT

Southern Boulevard Bridge – FDOT

Everything is so perfect that you would want to buy a post card of the future bridge. Unfortunately, if you dig a little deeper you will find that the beauty of the proposed bridge is only skin deep. As we pointed out in our prior post, the proposed Southern Boulevard Bridge fails at providing pedestrian and bicycle facilities worthy of its $90 million price tag. The bridge provides narrow 6′ wide sidewalks (compared to 8′ wide sidewalks on the Royal Park bridge to the north) and a painted white line provides protection to cyclists in the automobile shoulder. It gets even worse on the island, where 5′ wide sidewalks are provided. One a positive note, the latest drawings we received from FDOT now show a 7′ wide buffered bike lane for the island section. However, providing physically buffered bike lanes or widening the 5′ sidewalk to an 8′ wide multimodal path is a much better solution when you have a road pass through a linear park.

The proposed pergolas and overlooks on the bridge make one think that FDOT is really trying to provide first class pedestrian accommodations. However, the proposed narrow sidewalks aren’t in keeping with that vision. It appears that the design has become so focused on aesthetics that everyone forgot who the aesthetics were intended to benefit. All the landscaping, sophisticated bridge design, and Mediterranean Revival architecture doesn’t matter if you don’t create a space where people feel safe walking and biking.

So what could be done differently?

In order to create an environment where people feel safe walking and biking the sidewalks on each side of the bridge need to be widened to a 10′ width. This would allow for a bike lane and sidewalk to be placed behind the protection of a barrier. This configuration would be similar to this proposal:

Approved-for-the-cambridge-street-bridge-over-i-90 www.bostoncyclistsunion-org

Approved-for-the-cambridge-street-bridge-over-i-90 http://www.bostoncyclistsunion.org

Better bicycle and pedestrian facilities can be provided on this bridge for less money; in fact, approximately $1.15 million can be saved from the current design. An explanation follows.

In order to have 10′ wide sidewalks we must find four additional feet on each side of the bridge. Let us start with FDOT’s current bridge proposal. (Note the 6′ wide sidewalk on the right side with two people walking side by side. They must have shrunk the people!)

FDOT Proposal for Southern Boulevard Bridge over the Intercoastal

FDOT Proposal for Southern Boulevard Bridge over the Intercoastal

First, let us look at the automotive shoulder width in our quest to find 4′. At the December 11, 2008 public hearing an 8′ wide shoulder was proposed. Somehow since 2008 the shoulder grew 2′ to a 10′ width. An 8′ shoulder is perfectly adequate to park a broken down car or truck. In fact 8′ is the FDOT standard width for a parallel parking space. FDOT’s current version of its Plan Preparations Manual (PPM) figure 2.0.2 – Bridge Section allows for an 8′ shoulder for “low volume” undivided bridges. So we have found 2′ of the 4′ that we need for each side at no additional cost.

The next place to look is the 12′ wide travel lanes. When choosing lane widths it is helpful to consider context. The project is in an urban area with a traffic signal at the bottom of the bridge on the West Palm Beach side along with a 35 MPH speed limit. FDOT’s one size fits all policy requires a 12′ wide travel lane for an undivided roadway. A 12′ wide travel lane on an undivided highway makes a lot of sense if you have a 55 MPH rural two lane state highway such as SR-710 (Beeline Hwy.) in the northern part of Palm Beach County, but it makes absolutely no sense on an urban bridge with a 35 MPH speed limit. An identical City or County bridge in Florida is allowed to have 11′ wide travel lanes. Cities and Counties in Florida use a less stringent design manual based on national standards. If we go from 12′ to 11′ lanes we now have 3′ of the 4′ required feet.

Now that we have right sized the lane and shoulder widths we will look at the potential cost savings in the type of bridge proposed. The current FDOT renderings depict a post tensioned slab bridge for the main channel bridge. FDOT’s Draft Bridge Development Report (BDR) found that a conventional prestressed concrete I-beam girder was a feasible structure at a cost savings of approximately $2.16 million dollars over the post tensioned slab option. ($134 / sq.ft. prestressed I beam, $171 / sq.ft. post tensioned slab). (957′ long X 61′ wide X $37 sq. ft savings).

With the $2.15 million dollar cost savings we can add the missing one foot of width to each side of the bridge to provide our 10′ wide sidewalk. Using FDOT’s square foot bridge costs it is estimated that adding 2′ of bridge width for 1,080′ of conventional prestressed I-beam bridge ($134/sq.ft.) and 228′ of bascule bridge($1,494/sq.ft.) would cost $971,000. So a 2′ wider conventional prestressed concrete I-beam bridge would actually cost $1.15 million less than the current proposed post tensioned slab design.

In the interest of keeping the post short an analogy would be useful for these two types of bridges. The prestressed concrete I-beam is the Honda Accord of bridges – boring, reasonably priced, and reliable. The post tensioned bridge is the Ferrari of bridges. FDOT’s own bridge development report identified the post tensioned bridge as being higher risk than the prestressed concrete I-beam. The risk is due to the post tensioned slab bridge being a custom design and needing significantly more quality control in construction. Mistakes can happen. The 2007 collapse of the I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota was due to a design flaw.

Perhaps the best question to ask as a taxpayer is what type of bridge would a private toll company, interested in maximizing profit, choose? The answer would be the lower cost, tried and true durable bridge. There is nothing wrong with a signature bridge if you can afford it, but there is something wrong with a signature bridge that provides substandard facilities for the user of the bridge. It is important to get this right. If you are reading this post then you will probably not be alive when the future Southern Boulevard bridge needs to replaced.

Action item: Ask FDOT, Town of Palm Beach, and the City of West Palm Beach to provide wider sidewalks on the both the bride and the island. Go to the Public Meeting this Wednesday, March 11th at the St. Catherine Greek Orthodox church and ask for wider sidewalks.

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One thought on “How to make a great Southern Boulevard Bridge – Part #2

  1. Pingback: Southern Boulevard bridge: Another bridge that needs an underpass | Walkable West Palm Beach

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