Monday, March 31, 2014

The Capital Beltway Northwest Branch Bridge

125 feet above Northwest Branch stands one of the most important pieces of infrastructure in the Washington D.C. region.  It carries over 225,000 vehicles a day, and it forms a crucial connection between the east and west sides of the the National Capital Region.  It links people to their homes, jobs, and social lives, and it is a critical component of regional commerce.  It was completed in 1964, and it formed the final link in the Capital Beltway, completing the 64 mile circumferential highway.  It is taller than most buildings in the area, and it is by far the tallest bridge on the Capital Beltway.  Because of its unassuming appearance from the roadway, few of the quarter million people who travel across it on a daily basis realize its scale and importance, and some do not even realize they are crossing over a bridge.

It is the Northwest Branch Bridge.    

The bridge seen from the Northwest Branch gorge, looking north from the Oakview side of the creek.  The blue shape on top of the bridge is a large box truck that was driving over as I took this photo.  The truck provides a nice sense of scale.    

The bridge crosses over the Northwest Branch of the Anacostia River.  The bridge is located at one of the deepest parts of the gorge, just south of the fall line.  This location is a challenging place to build a bridge, but it is not arbitrary, it had to do with the routing of the Circumferential Highway (Capital Beltway).  Montgomery County was more developed than neighboring jurisdictions when the highway was being planned in the early 50's.  Most of the neighborhoods around Four Corners were developed by this time, as were other areas inside the Beltway.  The amount of development limited the possible routes the highway could take, resulting in things like the "Rock Creek Roller Coaster".  Had the Sate Roads Commission (predecessor to the SHA) wanted to chose a route that would have avoided the gorge, the location of the Beltway would have needed to be significantly altered, running along a northern alignment that would have passed through White Oak and Kemp Mill.  This detour was deemed to be too far out of the way, and with an Outer Beltway (what would become the ICC) on the books, the State Roads Commission did not want to use a northern alignment that would have put the two highways unnecessarily close together.        

The bridge and its approaches under construction in April of 1963.  Image from the United States Geological Survey. 

Construction started in the bridge in 1962, and the 26 mile section of the Beltway from Wisconsin Avenue to Pennsylvania Avenue (including the bridge) was the last segment of the Beltway to be completed in August of 1964.  The bridge gets overlooked because it is not as well known as the American Legion Bridge or Woodrow Wilson Bridge, both of which cross the Potomac River.  However, I think the Northwest Branch Bridge is much more impressive than either of those bridges from an engineering standpoint and an aesthetic one.  Construction of the Northwest Branch Bridge required serious grading work and earth moving (visible in photo above).  During construction, the massive open expanse of dirt contributed a large amount of silt and sediment to Northwest Branch (there were no environmental regulations regarding sediment management at construction sites back then).  The area that had once been sloping forested land had to be built up to form the approaches to the bridge.  When finished, the approaches were about 125 feet above the creek, and the bridge crossed the gap between them.        

Looking up at the bridge from below.  The road surface is at least 125 feet above the creek.  

The bridge is of a steel truss arch style of construction, a popular type of bridge commonly used on interstates during this era to cross large obstacles.  The bridge deck is made of concrete.  When originally opened, the bridge carried 6 lanes of traffic, three in each direction.  The median was decked over from the onset to comply with interstate standards and to make room for future widening.  By 1972, the Beltway in this part of Maryland had been widened from 6 lanes to 8 lanes, with the additional lanes being added in what had once been a wide wide grassy median.  The addition of the two travel lanes did not require any alterations to the structure of the bridge, since it had been built with future widening in mind.  The bridge underwent some form of upgrade in 1994 (the year inlaid on the concrete wall of the bridge previously said 1963-1994 before the most recent project two years ago), but I do not know the nature of this project.  Two years ago, the bridge underwent a complete deck replacement.  Many will remember that traffic was shifted on the bridge in stages to complete this operation.  The steel girders of the bridge were repainted during this project as well, to protect against rust and corrosion.        

The road surface during the bridge deck replacement project in 2012.  Image from Google Street View. 

The bridge now carries around 225,000 vehicles a day, and it is a very important part of the Beltway.  The structure bridges a gap in the region, both literally and figuratively.  Traffic travelling westbound frequently backs up approaching the bridge in the mornings due to the east west divide in the D.C. area.  Because Montgomery and Prince George's counties have done a poor job of attracting employers, many people that live in northern Prince George's County and eastern Montgomery County commute west to jobs in Northern Virginia each day.  This lopsided pattern is to blame for most of the traffic issues in the region.  The bridge is also one of the few road crossings over Northwest Branch, so both local and regional traffic get funneled on to it.  These factors make the Northwest Branch Bridge a critical piece of infrastructure.

The underside of the bridge looking west from the eastern abutment.  Photo by the author.

If you ever get the chance to hike on the Northwest Branch Trail under the bridge, I encourage you to do so. The bridge is a very impressive sight when seen from below, and viewing the structure from that angle is the only way to really appreciate its scale.  The embankments under the bridge are around a 35-45 degree angle, but it is possible to climb up them to get to the abutments.  The bridge is also very loud, and it shakes every time a large vehicle goes over it (I was holding on to the steel beams to take some of these photos, and I could feel the bridge moving).  It's interesting to think about all the people who drive over this massive structure on a daily basis without realizing what they just crossed.  

I would classify this impressive bridge as a "hidden gem" of Four Corners, and I encourage everyone to check it out.        


  1. This is a really interesting post, thanks! I'd love to see a map with the location of the trailhead marked to go walk around it and see views like your photos here.

  2. I grew up in hillandale and the northwest branch was our forest wilderness. My brothers and I actually crawled through those massive trusses from one side of the gorge to the other.