It's a very specific measure in the data you published, and the capacity to get a statistically unbiased measure pre-legalization would be difficult due to availability and protocol around THC testing.
Although I WHOLEHEARTEDLY agree that it's something that needs to be very carefully studied and monitored and legislated around... The answer to the question "are the roads more or less safe before or after legalization?" Is "They are equally safe within statistical margins"