DOing intervals on a trainer was most effective in my case. I used to think one had to push big gears on the road to go fast. This only leads to fatigue. It is like trying to bench press 300 lbs. You can do it a few times then you're spent. SO you benched 300 lbs 5 times, you moved 1500 lbs. If you bench 150 lbs, you can lift it 20 times. That's 3000 lbs of force. With the 300 lbs, it's doubtful you'll be able to do another set. With the 150, you will easily be able to continue lifting.
One season I actually trained to improve my cycling. I didn't count miles, how far behind I was left behind on group rides, what centuries I wanted to complete, it was all drills on the trainer and some long wekend rides.
My trainer workouts were high cadence intervals mixed with hard effort intervals. My workouts were along these lines.
10 minutes warm up
1 minute hig revs. (100 + while trying to maintain a smooth spin) Maybe a 39/15 combo, no bouncing!
3 minutes recoup pace
1 minute biggest gear ( 50 x 11). You want to puke by the end of this interval.
Then repeat the cycle.
After a couple of weeks, you will improve and feel stronger. I might alter the plan at times. Maybe drop the big ring workout a couple of gears and push HARD for 3 minutes. Then do a high rep interval after a 3 minute rest. Folloing the clock with planned recoup intervals is a good tool as it forces you to GO even if you aren't prepared. It's like listening to a coach telling you to GO NOW, rather than lolligagging around till you feel like it!
After a 45 minute session like this, you will be sweating like a dog but you wil know that you got a good workout. When I returned to theh road, I was shocked at how much my tech had changed. You will feel ike you want to do intervals and you have the ability.
But like others say, it doesn't make you faster. My opinion is that it allows you to carry a higher speed more efficiently with less fatigue.