Have you used this? If so, would you recommend it (ignoring the fact that it is expensive).
Sorry, no I haven’t used one.
@TTVegetable Per @splash reply, I’m in SW London and use some of the quieter lanes that are part of a Sporting TT course. Usually these are between 5-20km long with few interruptions. The TT’s will do several loops and you can do the same for the intervals.