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It's true that most bridges are "two directional."

But three way bridges are pretty rare, globally. I can understand why there wouldn't be many for rivers, but if bridges are designed based on the lie of the surrounding ground, why wouldn't there be a large number of non-river sites that would support such bridges.

On the other hand, three out of the world's bridges exist in Michigan (and only ten or so elsewhere in the United States). What is it about the land, topography, or other features of Michigan that cause it to have a disproportionate number of the country's and world's three way bridges.

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    $\begingroup$ Wouldn't a freeway interchange count? There are hundreds in Los Angeles where 2 lanes are bridges and join together off the ground then land before merging with a freeway. $\endgroup$ – Chris K Jan 20 '15 at 23:58
  • $\begingroup$ @ChrisK: Interesting point. If you count these, maybe such bridges aren't so rare after all, at least in California. $\endgroup$ – Tom Au Jan 21 '15 at 0:25
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Most bridges (and overpasses) are built to cross over something. With a few notable exceptions, most of these "somethings" are relatively long perpendicular to the desired crossing direction and fairly narrow parallel to it. Therefore a simple "two directional" bridge best meets the needs of the engineering problem.

Engineers always try to solve a problem in the simplest possible way to prevent introducing more problems than they have solved. If a simple two-way bridge solves the problem, then there is no reason to complicate the solution. Along those lines, vehicular traffic flow on a three-way bridge can be fairly complicated and may not easily facilitate high traffic volume. I believe that is why you will find more pedestrian bridges created in this style.

As for Michigan, I don't know. It's entirely possible that a structural engineer or engineering company in the area had a fondness for that type of structure and bid on those projects with three-way designs.

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Multi-way bridges are rare for river crossings for the reasons you describe; however, they are seen as a component of highway interchanges, usually when left entrances/exits are present or access ramps have been "braided" into an existing system interchange, or where a SPUI has been built over an existing divided highway. Viaducts over railroad tracks or highways are also built in this configuration on occasion.

An example of this is the intersection of Russel Blvd. and Gravois Ave. in St. Louis, MO, where a highway intersection is partially undercut by Interstate 55; another good example (in SPUI form), is at Hampton Ave and I-64, also in St. Louis.

Even more extreme forms of this can be found in Atlanta, GA, where the Downtown Connector freeway was tunneled under two intersections: Baker St NE and Piedmont Ave NE and Memorial Dr SE and Capitol Ave SE.

Finally, New York calls itself home to a true four-way bridge, the little-known Macomb's Dam Bridge in the Bronx (the four-way part comes on the east approach, where it intersects ramps from the Major Deegan Expressway that are elevated in order to pass over trackwork to the south).

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  • $\begingroup$ Yet another example from the Dallas Metroplex (of a single bridge structure carrying four four-way intersections!) can be found here. $\endgroup$ – ThreePhaseEel Apr 30 '15 at 22:36

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