I would like to know if less is used wrongly in this sentence. There are less accidents on California freeways per car than any other place.
It depends on who you ask. Some will say it must be "fewer" because "accident" is a countable noun. Others will say either is fine, because the rule is arbitrary and baseless and "less" has been used by excellent authors to modify count nouns for years.
That said, almost everyone agrees that "fewer" sounds right; a significant number of folks consider "less" to be a solecism. So my advice would be to go with "fewer".