A pictures of the lot, the approach from the direction of the trouble, and the surrounding neighborhood would be helpful. In the alternative, you could:
1. use landscape boulders,
2. appeal for speed humps across the roadway,
3. install a raised curb,
4. use concrete pyramid landscape cones,
5. do nothing,
6. fill the area with pea gravel, which is more user friendly to smooth out.
Are any of these appealing?
Your establishment of a semi-permanent object or objects (mail box, bollards, trees, fence, etc.) in the public right of way subjects you and your property to punitive damages if some idiot hurts themself or their vehicle there even due to their own neglegence. Go figure.