Thank you everyone for your suggestions! It was indeed a glitch (rather than a hidden trait, or not having been properly forgiven), so it gave me the final nudge to installing mods - I have to say, I'm glad I did, Overwatch cleaned 60 cars from my game in the first night, and it's running a lot more smoothly - and of course, my Sim is no longer late for school on account of sneaking to the bus instead of running.
There must be quite a few of these common glitches, easily fixable by mods - you have to wonder, if they're so simply fixed, why EA isn't doing it themselves (clearing up the town at night, small bugs etc). I mean, I'm incredibly grateful to Twallan and the other modders for spending their time to make all our games better, but honestly, my inclination would be to not mod if all else was equal - it's sheer laziness on my side here, I have to admit, I'd prefer to spend time simming that updating mods after patches etc. Anyway. Grumble over, and I'd like to reiterate my thanks to both the people on this thread, and to Twallan for the mods (seriously, those mods are great, they've made my game run so much better - can't recommend highly enough).