You need to be flippin' hard on the brakes to drop in without twatting the one at the front of the gap . . . the gaps most were leaving were reasonable for the speed they were doing . .. but not for a drop in to let some twonk in a van past unless you slowed down first . . . cue more headlamp flashing from twonk . . . and then if uphill, his lost momentum meant you pulled back out infront just in-time for the next down hill where he/she was up your chuff again . . .Says more about the pro drivers leaving no space than the restrictions
A commute I used to do regular for a time.