One of the more curious freeway to freeway interchanges in the USA is where US 53, I-35 and I-535 meet in Duluth, MN. This is a fairly compact interchange in a tight urban-industrial area. I-35 runs on a northeast to southwest path through here, I-535 goes southeast from here to the 'Blatnik' bridge to Superior, WI, and US 53 is on a northwest to southeast track and is multiplexed with I-535 over I-535's entire length (The section of MN 23 on this map has been decomissioned since the map was drawn). I-35 essentially serves as the 'Main Street' of Duluth, tieing the northeast and southwest ends of the city together and follows a very interesting path through the city while doing so. After dropping down a 3 km or so 6% grade into the city, it is neatly threaded through a mass of railroad trackage, under two Duluth, Missabe and Iron Range railroad iron ore docks (at the lower left corner of this map), past the I-535/US 53 interchange and through several tunnels under downtown to emerge on the city's northeast lakefront, ending at about 26th Av E (MN-(formerly US)-61) after wrapping around the Edgewater East Motel. It is a fascinating and very beautifull drive.
The MOST unusual feature of this interchange, though, is the fact that the ramp from NB I-35 to NB US 53 *INTERSECTS* the ramp from SB US 53 to NB I-35 and SB I-535/US 53 (the southbound continuation of US 53) AT GRADE. This intersection is about 15-20 meters above ground level - above two other levels of the interchange - and is controlled with a traffic signal. The ramps that actually connect I-535 to I-35 are fully up to I-standards and are unaffected by this intersection. US 53 takes two 35 km/h 'S' curves before the freeway ends about 800 meters north of the interchange. Although it is difficult to make out from the map, there is an interchange between US 53 and surface streets in the 'S' curve. US 53 then climbs a very steep hill as 'Piedmont Av' to the upper northwest side of the city.
Before the freeways were built, US 53 continued down the hill as Piedmont Av, and then southeastward to the Blatnik bridge (which opened as I-535 in the early 1960s and is decidedly *not* up to true I-standards due to narrow lanes, no shoulders and a sub-minimum speed limit) on the street that I highlighted with 'dashes'. There was a 'low level' US 53 toll bridge that predated the Blatnik bridge.
I consider it unlikely that there will be any major re-engineering of this interchange in the forseeable future that would eliminate the intersection, as topography and local access needs pretty much preclude it.
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