Given a choice between a street where the cars are stuck in 2km/hr traffic and I'm passing them with a less than foot (0.3m) gap, or a street with 70km/hr traffic where they're passing me with a 1 meter (3 foot) gap... the former feels a lot safer.
Admittedly these streets aren't usually close together (either in time or space), but I've certainly biked on both.
Still, imperfect data can be better than no data.
One potential issue with counting is that crashes aren't created equal. To reference back to the extremes I discussed above, if I crash when I'm going 5km/h and it's going 2km/h... it's fine*. If I crash going 30km/h with a car going 70km/h I likely have life altering injuries (or am dead, though I believe the statistics say I'm actually pretty likely to survive a collision at that speed differential).
I.e. fender benders between cars (and between cars and bikes, I assume) are common, but not really what we care about.
Not to say it wouldn't be an interesting map to make.
* I've never been involved in a collision, but I assume I'd be fine at these speeds and any damage minimal.
Indeed, that's a good point. My state maintains a map of reported crashes, and most of the dots on the map in my locale are on the highest speed roads. It seems like when the cars are going slower -- and there are fewer of them -- there are fewer crashes. And if the severity is less, like you say, then that's a compounding factor.
We're not NYC, where every street is packed with moving and parked cars. Most of the traffic is on the faster roads, and the cyclists tend to thread our way through the sleepy residential streets. That's good enough separation for me. The parts of town where bikes have to mix with cars, are where they focus more attention on bike lanes.
> It seems like when the cars are going slower -- and there are fewer of them -- there are fewer crashes
Yes, because the throughput of that fast, high density road is so much bigger. Subjectively it feels like it has something like 2x the amount of cars, and when we look at accident density we may very well correct by that factor, but in reality the difference in throughput is much bigger. Number of cars present at a given point in time x speed. That quiet road, it's close to having no car throughput at all compared to the big one, but it still sees the occasional accident.
I wonder if this can be predicted by a heat map of car crashes in your area. This is based on my private hunch that car crashes are a predictor of bike crashes. After all, if a car can crash into another car, or a stationary object such as a tree or a building, then it can crash into another bike. And the causes may be similar: Speed and inattention.
On such a map for my locale, the most crash-prone roads are exactly the ones that I instinctively avoid.