Yes, indeed, riding outside is fun. More fun than riding a trainer.
Furthermore, I came across this:
The podcast is aimed at ultra distance runners, so some of the language is a little different and it ignores the need for sprint finishing [which is important for some of the forum members]. However, it is a really clear presentation of interval training. The take away is this:
Intervals need to be done at ~ MLSS [ie, ~ 90% of VO2 max power]. A session should accumulate 20 - 40 minutes, in intervals of 1-3 minutes or of 5-10 minutes, about once or twice every ten sessions. Intervals do not have to be the same length. The length of time between intervals is not very important, though > 2 minutes is needed for immediate recovery; it’s not until 45 minutes or more that metabolism recovers from any one of these intervals, so recoveries of 2 - 5 - 10 minutes should be all right.
The presenter really stresses the lack of really tight experimental evidence about the efficacy of specific interval lengths and recovery times. Thus, he is giving a very relaxed view of the design of good interval sessions. And: he is giving a really good explanation of why training outside may be really effective, provided that we do not fixate on precise intervals lengths and recovery times.
If the OP is aiming the question at this kind of thinking about the efficiency, then this is a really useful podcast. Of course, it’s informed opinion, rather than tight experimental evidence. If the OP is aiming the question at time in zones, TSS and suchlike, then my previous post covers that.