Interesting, under 8 minutes costs more than 8 to 30 minutes
Probably to use it as a toll road instead of going to a terminal

http://www.dfwairport.com/parking/terminal/


0 min - 8 min $2.00
w/ TollTag $1.00
8 min - 30 min $1.00
30 min - 2 hours $2.00
2 - 4 hours $5.00
4 - 6 hours $7.00
6 - 24 hours $20.00
w/ TollTag $18.00