Interchange Index
I-20., I-59, and I-65
Birmingham, AL
Overhead Picture from terraserver-usa.com
Overview: The east-west freeway here is I-20 and I-59, near the eastern end of their 155-mile overlap. They both run west towards Tuscaloosa and Meridian, MS; to the east, they soon split, with I-20 running east to Atlanta and I-59 traveling northeast to Chattanooga. The north-south freeway is I-65. It comes from Mobile and Montgomery in the south, and travels towards Huntsville and Nashville, TN to the north. This interchange is Exit 124 on I-20/I-59, and Exit 261 on I-65.
Interchange Description: This interchange is a fully directional 4-way interchange, meaning that all ramps for left-turn movements (WB->SB, SB->EB, etc.) exit from and merge into the left side of the freeway. In the earlier days of freeway design, full directionality was much more common; now, left exits are being frowned upon and usually eliminated from new freeway and interchange designs. In this particular design, the number of bridges required for these left exits and entrances is reduced by actually having the lanes of through traffic cross over each other; in the middle of the interchange, through traffic on both highways is driving on the left side of traffic in the opposite direction. This provides for fully directional connections for all movements, while reducing the number of bridges needed from twelve (two for each left-turn ramp, four for the intersections of the through lanes at the center) to eight (four crossovers and four at the intersections of the through lanes).
Comments: Fully directional interchanges were once seen as a good idea; it would be logical, while approaching an intersection with the intent to turn left, to remain in the left lane. However, more modern interchanges shy away from left exits, because they require traffic to enter and exit the freeway (moving slowly) to do so in the fastest lane of traffic. If this interchange was to be redesigned and rebuilt now, it would probably be built as a four-level stack or a similar setup rather than something with so many left entrances and left exits.
However, this design is not totally without merit. A four-level stack interchange, which would probably be used if this interchange were built with today's design standards, requires four levels of grade seperation at a single point, while this interchange requires only three levels, which can be reduced slightly because all three levels don't intersect at a single point. (I-65's through lanes can drop slightly between the point where they go over I-20's WB lanes and under I-20's EB lanes; therefore, the difference between the highest and lowest level is slightly less than two levels.) The four-level stack would make much more of an impact on the local scenery, and would probably be noisier for the surrounding area because it brings the traffic well above treetop level. As a result of the extra grade seperation, the ramps for a four-level stack would most likely take up more room as well. It seems like this was built in a reasonably densely populated area; more land would have meant more families and businesses displaced and more money spent on the real estate.
Another note: An interchange identical to this one (with a few minor differences, such as which highway passes over or under which), can be found at the intersection of I-95 and I-695 in the northeastern suburbs of Baltimore. An overhead (Terraserver) photo can be found here. I-95 runs SW-NE, and I-695 (the Baltimore Beltway) runs roughly N-S.
Corrections? Suggestions? More information is always welcome.
Suggestions for more interchanges to cover on this site are great too.
Contact the author, Dan (known as DanTheMan on misc.transport.road):
twowheel@email.com
