Are you sure the interchange numbers correct? It seems unlikely that the "very large" interchange is smaller than the "large" interchange (300 x 240 versus 300 x 280, respectively, according to your numbers). Also, what is the "road interchange", and is it really the largest (300 x 400)?
For those of you who have experience with highways, are there simple on-ramps/off-ramps available which do not require elaborate interchanges? Could you elaborate on the options available (e.g., explain the differences between the interchange types), and could you approximate the size of on-ramps/off-ramps, if they exist?