Over most of the trip from Menlo Park to Seattle we were charging during the day at 40 amps and 240 volts. This was putting between 30-35 miles of ideal range back into the car per hour, more when it was cool, and less when it was hot. We usually stopped to charge for 2 to 4 hours.
A Tesla HPC ( 70 amp 240 volt ) charger can put between 60 and 70 miles per hour back into the car. If there had been 4 or 5 of these along the I5 corridor in key spots we could have stopped for midday charges and had charge times of 1 to 2 hours instead of 2 to 4. To get the most out of these chargers, they should have a small shade structure to let your car charge in the shade.
Its hard to say you need a charger much faster than this, even for long trips. It is really nice to be able to stop and sit down for a decent meal or stretch your legs. Having three 70 amp charge stops along the way would mean you could easily cover 500 miles in a day in a Roadster with 3 charging stops of about 1.5 hours each and never get very low on charge. I’ve done 700+ mile drives in a single day, and I’d trade most of them for a more relaxed and fun drive in the Roadster that takes an extra half day.