"It looks like they had a clever 'starter' motor--a big-legged sprinter pushing a thin inline skater pushing the vehicle. i reckon they might get him up to nearly 40 mph before he had to start putting his legs into it."
Closer to 15 or 20mph. The pusher is only allowed for 15 meters and the skater is not allowed to input power.
Note that the low altitude record is "only" about 62 mph.
IHPVA - International Human Powered Vehicle Association
The reasons for choosing this site are a combination of the reasons previously stated. Long run up to the 200 meter timing traps(5 miles), relatively flat(.66% slope), smooth road at a relatively high altitude for less air resistance.
If the new speed record were equivalent to 100mph at sea level, I'm fairly certain that Delft or Sam would have found a suitable place to race so that they would be able to say they were the first human powered vehicle to reach 100mph.
JJ