After some delay Arkansas DOT acceded to a legislative request to raise the speed limit to 75. The is open to comment by state residents (only) until December 13. Stripped of irrelevant material the 33 page speed limit report has five pages of content.
One graph says the average speed on rural interstates has been about 53 mph and the 85th percentile speed 71. Another gives the corresponding figures for urban freeways as 48 and 59. Rural multilane highways other than freeways fall in between. Rural two lane roads have 85th percentile speeds in the 60 to 65 mph range.
If these figures were accurate, the proposed 75 mph rural and 65 mph urban speed limits would be justified. The speed limit should be set so most drivers are legal. I don’t believe they are accurate.
On a typical American rural freeway the 85th percentile speed is 70 to 80 mph and the median speed is 5-8 mph lower. The Arkansas measurements are nearly 20 mph apart. On urban freeways the measurements are 11 mph apart.
I believe Arkansas DOT did not report the speed of free-flowing traffic (as required by the Manual On Uniform Traffic Control Devices), but rather an average including all types of traffic flow. Cycles of congestion and weather will increase apparent variance in speed and depress all percentiles, explaining Arkansas’ graphs. For example, if drivers go 80 on a clear day and 50 during a busy, rainy holiday weekend, the combined speed distribution will be very broad, from 50 to 80. Posting a 70 mph speed limit based on the aggregate data would create a very high violation rate during the times when speeding tickets are written. In this case the speed limit should be based on the 80 mph free flowing condition, not the 50 mph traffic jam.
The study also recommends posting 65 on rural four and five lane roads other than freeways, with increases not allowed. Such roads tend to be posted 65 or 70 except on the East Coast.
The standard speed limit on rural two lane highways would remain at 55, but increases to 60 would be allowed. This is based on data showing the speed limit should be 60 in hilly terrain and 65 in flat terrain. Such roads tend to be posted 60 or 65 except on the East Coast, but some states post 70 or 75.
If any readers in the South have laser guns, a trip to the nearest freeway could yield interesting data. While it’s possible a 75 mph speed limit is justified, I find the report unconvincing. I’d like to know what’s really going on out there.
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