There’s a story about a former TDOT commissioner coming into the office one morning. He immediately called the chief engineer into his office to get an explanation for a driving mishap from the night before. As the story goes, the commissioner was driving down Briley Parkway toward I-40 in Donelson. Somehow he ended up going toward Wilson County instead of west toward downtown. (And we all BAD THINGS happen in Wilson County.) The explanation for this sorry state of affairs is one of the rare times having a good sense of direction can get you into trouble.
The interchange at Briley and I-40 is what we call counter-intuitive. If you’re going south on Briley then your sense of direction will tell you to stay to the left if you want to go west toward downtown, because that’s how it looks on a map. Unfortunately, the roadway signs say the opposite. I’ve had the same problem at that interchange because I navigate based on an internal map. My wife, on the other hand, has absolutely no sense of where things are in relation to each other so she just reads the signs and has no problem.
Fortunately for the commissioner, he had the authority to demand an explanation for why that interchange was built so that you have to veer right to turn left, and left to turn right. For those of you middle Tennesseans who don’t have the TDOT chief engineer on speed dial, I’m here to help. (Technically, this is speculation because I didn’t talk to anyone on the design team, but I have enough experience to figure it out.)
The essential problem is lack of space. Interstate interchanges take A LOT of land and urban Nashville is very short on land. If you look at the aerial view the problem jumps out pretty quick. I’ve enhanced the map our friends at Google provided by adding color coding to show the ramps in question. The blue line shows southbound traffic turning right to go east, and the pink shows southbound traffic turning left to go west.
To be completely specific, the problem is the different speeds of the traffic coming from various places. When designing an interchange between two high speed roadways you have to provide a specific amount of distance for acceleration so that the cars on Briley are going approximately the same speed when they join through traffic on I-40. Traffic on Briley has to slow way down to make it through the 90° turn and get facing in the right direction to merge with I-40, and this requires a fairly long ramp to get up to speed before merging with 70 mph (to be more honest, 80 mph) traffic .
Seems like no big deal. There are lots of interchanges in this country that manage to merge traffic in an intuitive way. There’s one big asphalt problem here. One of the runways from Nashville International Airport pushes right up beside I-40 immediately next to this interchange. Apparently a tunnel was briefly considered, but that idea was wisely killed as soon as management took a look at the cost estimates.
Since there was no room on the southeast side of the interchange, the eastbound ramp had to curve west to get some distance before then going east as intended. This added roadway length for the cars to start getting back up to speed. The westbound ramp had to remain as close to the main line as possible in order to keep from obliterating several industrial buildings.
So in the short version, the answer to the commissioner’s question was “Because it was cheaper.” Though I suspect quite a few drivers grumble about it every day.