Solo I is a race against the clock rather than wheel-to-wheel against other vehicles. Unlike Solo II's, which are usually conducted on parking lots with minimal obstructions and have an "understood" speed limit of 65 mph, Solo I's are often held on real racetracks with no pylons to artificially keep you under a given speed. Although the safety equipment is more extensive than that required to run Solo II (because of the higher speeds and closer proximity of immovable objects), many Solo I competitors race street-legal cars. Solo I does NOT require a dedicated, single-purpose vehicle.
the average Solo I offers five to ten sessions per weekend, with around five minutes of track time per session. There are multiple classes in which street-legal cars (with a roll bar, a five-point harness and a fire extinguisher) are extremely competitive, and the fastest of the Street Prepared cars reach almost 150 mph on the front stretch at Roebling Road.
[This FAQ taken from 'Solo I for Fun and Profit,' by Butch Kummer]
