Continuing past main road turning right? Absolutely! (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

definition: the past imperfect tense is used for actions that were continuing in the past MI=the M1=”main road” reversed/”turning right”; plus PERFECT (e.g. said in reply to a suggestion)=”Absolutely” edit thanks to Quirister in comments: reversal indicator “turning” and PERFECT from “right? Absolutely”