Downhill, yes xD But there's no way you can achieve the same speeds on level ground.
Midland is almost flat, and I could reach 30 if I wanted (I should get new wheels first, though). It really depends a lot on the skater, and as I am a born sprinter, I generally don't have too much of a problem reaching that speed (I know this because cars on Midland go about 35 - 40, and they just barely pass me sometimes). Physically, it is possible, but you need these to make it work well:
- Nice bearings.
- Strong legs.
- Agility (you need to get going this fast to understand just how agile you need to be, even if it doesn't seem like it).
- The ability to detect tiny changes in the road and react almost immediately.
If the above are ignored, you'll fall flat on your face at high speeds. Oh, and make sure you have sufficient rubber left on your wheels (I don't... >_>).