unless you have a barrier a car will still damage the planter. Could you put down white painted large stones to mark your boundaries. That is what people have done in the village near the church to get over this problem.

or could you put up a fence?

Or concrete posts?

Agree as above you need something more substantial than a planter. Are you able to install raised kerb edging near your plants?

I'm with Cammomile - A couple of concrete posts should do the job, depending on the length of your hedge.

I would suggest a raised bed made of stone or concrete blocks with the hedge planted in this. We have this, minus the hedge part, as out roadsider boundary as there is no pavement. It is amazing how much respect car drivers give it!

railway sleepers would work too or concrete blocks 9" x 18" buried half way down long ways . bricks after a while can start breaking down but blocks wont and they would be cheaper in the long run .

If they are in planters because there is no bed down the drive then Snoops's row of bricks or SBG's stones that the car hits before it reaches your planters sounds a good idea.
You can get reconstituted stone or concrete planters that would do the car a lot of no good, but they are not cheap and are extremely heavy.

Thanks to all who replied. The comments were all helpful. It's a pity that car drivers don't respect private property. Thanks to all,

I'm all for bringing back caltrops...........

or hanging lol . people do however care about the paintwork on there car and its not that easy to see . the bigf painted stones would do realy well . its the old fashioned things that work best sometimes for a good reasen lol .

you could lay a few planks of wood with nails knocked right threw and turned over lol not realy ,.

Well - they'd only do it once, wouldn't they?

apserlutly lol