Dror Bar-Natan's Image Gallery: Knotted Objects:

An interchange near Baltimore

An interchange near Baltimore
The interchange of I-95 and I-695,
northeast of Baltimore.

In America it is much easier to make right turns than it is to make left turns. This is clearly seen in the traditional "clover leaf" design of highway intechanges, as seen on the right.

In England, on the other hand, it is easier to make left turns than it is to make right turns. The lovely interchange of I-95 and I-695, northeast of Baltimore (shown above and below), combines the advantages of the two systems - it has an outer layer of American (right) turns, followed by a braiding of the lanes, followed by an inner layer of English (left) turns!

(A related good reason to build interchanges in this way is disscussed in a problem by Bill Thurston)

(The theory is nice. But I got an email from a driver who often travels through this interchange reminding me that merging from the left, using your right side mirror, into left lane high speed traffic, can be a scary proposition. Perhaps that's why there aren't many more of those.)

A clover leaf interchange
I-95 and Maryland RT 43
(EROS Data)

Notice that the lanes of I-95 and of I-695 are braided in a non-trivial way, proving that the designer of the interchange cared about elegance. Notice also that the center square is not weaved; this would be more elegant, but also less efficient.

Here are some high-altitude aerial photos of the intechange: (Data available from U.S. Geological Survey, EROS Data Center, Sioux Falls, SD)

Click to enlarge!
Infra red color aerial
Infra red color image,
20,000 feet, April 1989
Visible light black and white aerial
Visible light black and white image,
20,000 feet, February 1998

The best image I know of is at Google Maps, but if I understood their "terms and conditions" correctly, I'm not allowed to include a copy here. After clicking here you can move around and zoom using google's controls.

I'd be delighted to post better photos! Contact me if you have any. I'm at drorbn@math.toronto.edu.

Added April 5, 2008. Dylan Thurston reports that in the name of progress, this interchange is about to lose its lovely topology. See 1 and 2.

For references and technical assistance, thanks Chris Beaumont!
For the idea to look at google maps, thanks Greg Friedman!