Relating to several terms in case about reversing on motorway (9)

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

That’ll be x+y-z for example. I did wonder if several was a bit too many for three, but let’s not quibble. Case is TRIAL and it’s about ON reversing to give NO and M(otorway). Lost time working with “in case”, and terms to suggest word endings. [tilbee correctly points out that we need a second I in the wordplay, so it’s not just a Motorway but the M1.]