That's not how it works, actually. Progressive's Snapshot discount doesn't take speed into account at all.
The three things they look at are:
1) How often you drive (miles)
2) What time of day you drive
3) Number of hard stops
I noticed that driving with a Snapshot for 6 months I became a lot more careful of hard stops. I gave other cars more space and drove much more defensively, even though I'm a very defensive driver already.
I think it's safe to assume that an insurance company is interested in metrics that actually correlate well to safe driving, since their business literally depends on it. They want to give the discounts to people who are actually less likely to get into accidents.
Progressive isn't the government. They don't want to just look like they're doing something about a problem. Their bottom line actually depends on it.