South Dakota's recent speed monitoring report (dated 5/28/96) shows the following numbers for the Interstates:
| 1992 | 1993 | 1994 | 1995 | 1996 | |
| Average (50th percentile) | 65.6 mph | 65.4 | 66.2 | 66.0 | 68.3 |
| 85th percentile | 72.3 | 72.0 | 72.5 | 72.8 | 76.0 |
Yikes! Could it be? A speed limit set to the point where it is the safest possible speed, and furthermore, that most people aren't exceeding it?
Then there are the two-lane roads, once posted at 55, now 65:
| 1992 | 1993 | 1994 | 1995 | 1996 | |
| Average (50th percentile) | 57.6 mph | 56.4 | 57.8 | 58.1 | 60.7 |
| 85th percentile | 63.8 | 62.4 | 63.8 | 64.4 | 68.6 |
Look at the 85th percentile numbers. When the limit was 55, people were going 7-9 mph over the limit. Now that it's 65, they're going 3 over. For the Interstates, the differences are less but still visible.
Here's something that makes you think. Note that South Dakota's speed-limited drivers are driving faster than the Montana drivers who have no speed limits. About 15% of South Dakota drivers exceed 75, but only 8% of Montana drivers do. Interesting, no?