If you aren't fussy about it, you can train without a computer.

The key is to do intervals, albeit in a very informal way. Ride along for a short period; sprint all out until you reach a specific milestone up ahead or a turn; go back to normal speed; sprint again, etc.

Runners do it all the time, it's called "fartleking." In fact, a lot of runners don't like to use HRM's, they prefer to "trust their feelings."

And of course, you can ride without a computer on your low-effort / recovery days.

Strictly speaking the Joe Friel-type stuff might produce the maximum benefit -- only if you can stick to it. IMO, the "best" system is one that works for you.