I've always been curious about the Sam Rayburn Toll intersection with US 75. They built direct connecting flyovers coming from the east and an overpass over Medical Center drive. This makes me wonder if they plan on continuing the freeway even further to the east, possibly to 380? Anyone heard of any plans like this?
http://maps.google.com/maps?q=sh+399&hl=en&ll=33.161301,-96.63763&spn=0.01922,0.042272&sll=33.163601,-96.629992&sspn=0.01922,0.042272&t=h&gl=us&hnear=SH-121+Northbound+to+US-75+Northbound+Feeder,+McKinney,+Collin,+Texas+75069&z=15
Haven't heard of any plans, but any extension would take decades since you're looking at building an extension through the Wilson Creek and Trinity River bottoms and the resultant red tape. The flyovers were probably built for better hospital access, and the spur is a divided highway that leads into downtown McKinney, so there is volume there.