More results of that:
1. Car insurance rates would drop because claims and wrecks would plummet.
2. Health insurance rates would drop because fewer people would be killed and injured in car wrecks.
3. Prices of cars would drop because carmakers would not be required to continually invest in better and better safety devices in cars because of the already mentioned lower death and accident cases.
4. More hybrids would be sold, driving down the price of hybrids and replacement parts.
I'll think of others and post them as we go along......

OK MORE:
5. Air would be cleaner, lowering cancer rates.
6. Global warming due to vehicle CO2 emissions would slow.
7. Highway repair costs (and maybe hwy tax rates) would drop because lower speeds do less damage to the roads.