Solo could include other people in the car, but only one drives on the trip.
If you tell me you drove 20 hours straight, that's a lot different if it was just you doing that or you and 9 others driving for 2 hours each and sleeping the rest.
Maybe even break it down to
Single Driver
2 Drivers
3+ drivers (because at that point sleep isn't really the driving issue anymore, it's just how long you're willing to keep driving)