Although it can be street legal, the Radical was designed to perform best on the track.
In all fairness, there is a street pack available for pretty much every car which Radical makes that can convert it to street legal status. But SR3 SL is the first model which the company has produced which was designed for specifically for street use (the "SL" in the name stands for "street legal"). That is to say, it wasn't designed for that at all, it is still very much a track car, but Radical wanted to be able to offer you the option, whether it was a good idea or not. The important thing to remember about Radical is their race heritage.
Sure, Caterham has strong ties to F1, but the only car in this series which can be said to be directly based off of a top-tier race car is the SR3 SL. That car is the SR9 LMP2, Radical's most earnest attempt at an endurance racer. This was a car, built in 2005, which was not intended to compete directly with the big Audis, but rather in the smaller LMP2 class of prototypes. It didn't fare all that well, but it did complete not only the race at La Sarthe, but also all of the races in the Le Mans series. For new constructor, and especially a small one which doesn't have Audi's budget, this was quite an accomplishment.
Radical has also participated in the series using purchased cars, and it is the lessons learned from racing themselves which they bring to building their own cars. Their other models are even more track focused than the SR3 SL, and have earned a reputation for being supremely good giant slayers. Most models are powered by the Suzuki Hayabusa engine, a high-revving piece of reciprocating insanity which is never more at home than it is on a track. But for the SR3 SL, Radical was looking for a flagship model which you could conceivably use to drive to the track as well.
So the SR3 SL has a heater, an interior light, power mirrors, and a 2.0-liter turbocharged Ford four-cylinder engine. The engine produces 300 horsepower and is far more comfortable with city traffic than the Suzuki engine is. These refinements are nice, but they take virtually nothing away from the fact that this is fundamentally a track car. There is no roof, and a big roll bar and rear wing draw the eye immediately. One has to climb to get in, and a proper racing harness straps you into a racing seat. The LMP heritage is also plainly obvious to anyone who has seen any such cars.
There is a heavy clutch which is painfully difficult to operate in stop-and-go traffic, although the paddles take over sifting duty once you get it moving. The result of all this is that the car is fast. We're talking about a 3.4-second 0-62 time, quite a bit faster than that of the Focus ST which the engine is borrowed from. And if all of this talk of race heritage and fancy transmissions has you thinking that the SR3 SL is expensive, well, you're right. But with a price just over $100,000, it's fairly middle-of-the-road for a track car. And even if you'll pay more than you would for a base-trim Atom or a Caterham, those other cars just don't have the pedigree of the Radical.
Another important factor, when it comes to totaling up what the real cost of running the car would be, is that Radical's cars don't just race, they are endurance racers. Their cars are capable of withstanding terrific amounts of punishment, and aren't some delicate formula car which needs all of its components replaced after every lap. The bad news is that it really doesn't work on the street after all, and a Caterham 7 would really be a better way to go if there's anything resembling a speed bump along your planned route. Even the Caparo T1 has a suspension which can be raised for bumps, but you're just plain out of luck in a Radical.
This on top of the other problems associated with driving a track car on the street, including but not limited to becoming a speed bump of a different sort for some cranked-out truck driver somewhere along I-80. It's easy to think of the SR3 SL as being basically the same as any other street-legal track car. The combination of a lightweight body and a reasonably powerful engine seems like it would be simple enough for just anyone to do. That might even be true, but it takes real race experience to make the sum of all those parts work well together, and Radical has put in the track time.
So although, at a glance, the Radical doesn't seem to have any one thing that sets it apart from the rest, their ability to put together an excellent package makes their cars a force to be reckoned with.