I've recently moved yards where the only hacking available is on roads, though I'm unsure of how my mare will react to the cars. How can I prepare her so she feels safe on the road?

See if you can ride her around your own car/mum's car so she gets use s to her reflection and then progress to a moving car behind and beside her. Also ride with a buddy (and a quiet one too) so she gets confidence from them.
totally agree with having a buddy. but maybe take her for a hand walk down the road a ways so you can get an idea of her reaction before you ride in the situation
Hey, if I were you I would do almost a full schooling session at home, then ask someone to go with you on a horse that is use to traffic and just got for a 10min wonder after your ride so she's a bit tired. She'll be less likely to react to the traffic and a support buddy that will show her that traffic isn't something to worry about :)
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