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Just to put the 85th percentile speed in perspective -- although you may have already seen this stuff -- let me show most data "pre-NMSL".
The "Traffic Speed TrendS reports of the Federal Highway Administration (FHwA) for the years just before the 55-mph National Maximum Speed Limit (NMSL) may be of interest.
Two participating States had no speed limit, eight had 75-mph limits, 22 had 70-mph, two had 65-mph, three had 60-mph.
AVERAGE SPEEDS OF FREE-MOVING VEHICLES AND PERCENTAGES OF PASSENGER CARS TRAVELING IN EXCESS OF VARIOUS SPEEDS:
ON LEVEL, STRAIGHT SECTIONS OF THE COMPLETED PORTIONS OF THE RURAL INTERSTATE SYSTEM FOR 1973
(NOTE: speeds were measured in the summer, before the embargo and oil price crises in the fall.)
| STATE | Avg | %>55 | %>60 | %>65 | %>70 | %>75 |
|---|---|---|---|---|---|---|
| (No Speed Limit) | ||||||
| Mont. | 70.2 mph | 93 | 85 | 68 | 48 | 26 |
| Nev. | 70.1 mph | 97 | 92 | 78 | 54 | 28 |
| (75-MPH) | ||||||
| Ark. | 65.8 mph | 93 | 82 | 60 | 26 | 8 |
| AZ | 69.1 mph | 92 | 82 | 67 | 46 | 22 |
| Iowa | 67.0 mph | 92 | 76 | 60 | 28 | 11 |
| Kans. | 69.4 mph | 97 | 91 | 76 | 52 | 18 |
| Nebr. | 69.8 mph | 96 | 89 | 75 | 51 | 19 |
| N.D. | 69.2 mph | 95 | 86 | 69 | 47 | 21 |
| S.D. | 65.3 mph | 87 | 72 | 50 | 26 | 7 |
| Tenn. | 62.0 mph | 86 | 69 | 32 | 11 | 1 |
| (70-mph) -- slowest, fastest, and Ohio speeds | ||||||
| Flor. | 65.0 mph | 93 | 80 | 53 | 23 | 6 |
| Ohio | 66.6 mph | 96 | 89 | 63 | 31 | 6 |
| N.Mex | 69.0 mph | 94 | 86 | 72 | 45 | 19 |
| (65-mph) | ||||||
| Penn | 61.8 mph | 86 | 60 | 26 | 5 | - |
| N.C. | 65.4 mph | 95 | 78 | 44 | - | - |
| (60-mph) | ||||||
| Conn | 65.6 mph | 100 | 84 | 49 | 15 | 0 |
| RI | 62.7 mph | 88 | 54 | 35 | 11 | 6 |
| NJ | 59.5 mph | 85 | 56 | 12 | 3 | 1 |
I estimate the prevailing (85th percentile) speeds as following:
| SPEED LIMIT | Prevailing speed | Average speeds |
|---|---|---|
| -NONE- | 76- to 77-mph | 70-mph |
| 75-mph (*) | 72- to 76-mph | 65- to 70-mph |
| 70-mph | 71- to 76-mph | 65- to 69-mph |
| 65-mph | 66- to 69-mph | 62- to 65-mph |
| 60-mph | 64- to 70-mph | 60- to 66-mph |
I conclude there's little point in a limit lower than 70-mph or higher than 75- mph.
People may feel more comfortable driving faster nowadays -- it was 20 years ago this summer the speeds were taken.