At https://www.fhwa.dot.gov/modiv/programs/intersta/docs/interchange_design_promptlist.pdf https://www.fhwa.dot.gov/modiv/programs/intersta/docs/interc... I see

At https://www.fhwa.dot.gov/modiv/programs/intersta/docs/interchange_design_promptlist.pdf https://www.fhwa.dot.gov/modiv/programs/intersta/docs/interc... I see the short comment > At service interchanges it is desirable to design the interchanges with the crossroad above the freeway due to: > - The crossroad above the freeway results in longer sight distances to the exit ramp and gore area. > - The crossroad above the freeway allows gravity to assist the operation of both accelerating vehicles (the on-ramp has a down-grade) and decelerating vehicles (the off-ramp has an up-grade). In addition, the resulting grades generally provide longer sight distances. But that's the main thing I can find right now.