As I explain over at MLP&R, these numbers are a bit "baked" by leaving out California:
http://lawprofessors.typepad.com/marijuana_law/2018/02/new-report-of-notable-increase-in-pedestrian-fatalities-caused-by-drivers-in-marijuana-legalization-.html

California saw a major decrease in pedestrian fatalities for the first six months of 2017 (-81 deaths; -18.7%). The other legalization jurisdictions had a +30 increase in deaths. Moving California from the "all other states" category into the legalization category --- as it properly belongs --- would actually lead to the legalization states having -51 deaths from '16-17, and thus having collectively a much larger percentage decrease in pedestrian fatalities (about -8%)than the "all other states" group.

California did not have legalized recreational pot during the first six months of 2017, which is the study period. It means nothing that pedestrian deaths were down in California during that time. Recreational pot became legal in California on January 1, 2018.

Though I would defer to your understanding of California law, Michael, I believe Prop 64 immediately became effective in Nov 2016 making it legal to possess an ounce of pot and up to six plants in your home. What started on Jan 1, 2018, were legal regulated sales in store fronts. So, technically, California did have legalized recreational pot during the first six months of 2017.

Moreover, the situation in California in the first six months of 2017 is comparable/identical to the situation in Maine, Massachusetts and Nevada, all of whom also voted for legalization of adult use in Nov 2016. Nevada did not have sales in stores until July 2017 and Mass and Maine still do not have stores open. If it makes sense to kick California out of the discussion/analysis here based on when storefronts open, it also would make sense to kick out Maine, Massachusetts and Nevada.

I am not claiming marijuana legalization explains California's drop in pedestrian deaths, and I think these data are important to follow for many reasons. But it is dishonest to assemble the numbers this way in order to push the notion that the first part of 2017 saw an increase in pedestrian fatalities in states which have legalized recreational marijuana. In fact, when California is properly included, there was a cumulative decrease.

Sorry to spank your monkey professor, I only live in California's state capitol, but perhaps you will believe the New York Times:

https://www.nytimes.com/2018/01/01/us/legal-pot-california.html

Since you are nearby, Michael, you can talk to some state lawyers to explain to you how this work: pot became legal for consumption and home grow right after Prop 64 passed in 2016. Regulated stores selling pot did not start in Cal until Jan 2018. But the same was the case for Maine, Mass and Nevada in first half of 2017 --- all had, like Cal, legalization but no stores yet open.

I do not think this should be too hard of a concept for you to understand if you try hard. But feel free to ask for help from others in the state capitol. I think they will be able to confirm legalization that happened in 2016, but stores were not allowed to open until 2018. Like Maine, Mass and Nevada, California was a legalization state without stores in first half of 2017.