Origin and prevalence of x5 advisory speeds

[hbelkins]

Lately I've been wondering why most advisory speeds are x5's. I know this predates the national 55 mph speed limit. In Kentucky, it's rare to see an advisory speed ending in 0.

I'm curious as to how this practice came to be and why it's so prevalent? Why are so many curves signed for 35 or 45 instead of 30 or 40?

[SkyPesos]

I've seen a bunch of 60 mph advisory speeds on US 59 coming back from Houston 2 days ago.

[Max Rockatansky]

They are sparingly applied in California compared to other states.  There are some segments of mountain highway where I don't even recall seeing them on Caltrans maintained roads (along with regular speed limit signs).

[LilianaUwU]

In Québec, every advisory speed ends in x5 while every speed limit ends with x0. It makes things less confusing, I guess.

[SkyPesos]

In Québec, every advisory speed ends in x5 while every speed limit ends with x0. It makes things less confusing, I guess.

x5 km/h speed limits aren't that common anyways. I've only seen x0 with a few special cases (like 5 km/h for a parking lot).

[STLmapboy]

Missouri loves its 35mph advisory speeds on rural two-lane roads, but there's plenty of 30 as well.

[roadfro]

Lately I've been wondering why most advisory speeds are x5's. I know this predates the national 55 mph speed limit. In Kentucky, it's rare to see an advisory speed ending in 0.

I'm curious as to how this practice came to be and why it's so prevalent? Why are so many curves signed for 35 or 45 instead of 30 or 40?

This is a good question.

The question could be extended to speed limits as well. In Nevada, the vast majority of speed limits end in a 5. Most urban speed limits are 25, 35, or 45, and most urban freeways are 65. Many rural highways are 55 or 65. School zones, by state statute, are either 15 or 25. (The biggest exceptions to the 'ending in 5' rule are certain stretches of rural interstates signed at 80, rural US routes signed at 70, and parts of McCarran Blvd in Reno signed at 50.)

Typically, NDOT doesn't sign an advisory speed unless it's at least 10mph lower than the speed limit. So I think many of our advisory speed limits ending in 5 feed from the speed limit. Off hand, I can't think of an instance of an advisory speed ending in 0...


One plausible explanation: I seem to recall that many older vehicles had speedometers where the major marked increments were those ending in 5, i.e. you'd have the hash marks at every 5 mph increment but only those ending with a 5 had a number next to them–and during the national 55 speed limit days, 55 often had a special mark. (By contrast, the speedometer in my 2015 Toyota marks the speeds ending in 0–I feel like an older Toyota my family had they ended in 5, but I can't be certain.) So using more speed limits and advisory speeds ending with a 5 on the roads may have been easier for drivers to comply with based on their vehicle.

However, I wonder if this was vehicle manufacturers influencing speed limit policy, or typical speed limits influencing vehicle manufacturers, or is completely unrelated. If they were related, what prompted this convention and when did it change?

[Bitmapped]

Advisory speeds ending in x0 mph and x5 mph are both common in WV.