What is the point to having a measured mile along the highway? Like, is it any more accurate than the markers already in place to call out tenths of a mile on the very same interstate??? Can someone please answer that? it’s been bugging me!
- Jinx
The mile markers are basically roadside addresses that let police, rescue vehicles, and road crews find a precise location.
Measured miles are used to calibrate vehicular speedometers and odometers.
Even the address markers must be placed with some accuracy, or they’d be meaningless. Especially those highways where exit numbers go by the mileage, it seems pretty accurate. I’ve done my own odometer testing…
While the standard highway mile markers may be reasonably accurate in most cases, you have no guarantee that this is the case in a given instance. They are only there for reference. If the point where such a marker is to be placed happens to be occupied by an off ramp or some natural obsturction, it will be placed up to several yards from it’s actual intended location, or possibly skipped over altogether. Measured highway miles, on the other hand, are guranteed to be one mile, regardless of surroundings or other factors. The whole point of calibration is to have a precise standard against which to make comparisons, otherwise, it’s pointless.
OK, but measured miles definitely pre-date digital odometers. How well could you access the accuracy of your odomoter if your watching the tenths dial spin slowly an need to know by eye when it’s made exactly one revolution. I’d think you’d get better accuracy by comparing over ten miles using the regular roadside markers wouldn’t you?
Accuracy aside for a moment, what is the purpose of checking your speedometer? If its wrong, what then? Are they adjustable? If so, wouldn’t the mechanic have some sort of calibration tool? Would he just take your word that it was 1/10th of a mile slow?
I just don’t see the point, I guess.
Well, they are probably there mostly to pacify the obsessive compulsive, but there is a kind of logic to having them.
Most speedometers run off the gearbox: i.e. they count the number of revolutions that the wheel does. Fine, except that the radius of the wheel is not a constant. The odometer and hence the speedometer (runs off the same cable) will give an inaccurate reading if you put non-standard tyres on the car, replace with a different diameter wheel or just have your tyres wear down a bit.
Now, if you guage your distance against the roadside markers then you can discover for example that your odometer reads 1.5% out. That might be a useful fact if you want to set your cruise control just shy of radar-trap speed.
What’s a “measured mile”? From the above, it’s obvious you’re talking about something that’s distinct from the highway mile markers that are all over the place. Where would I find these “measured miles”? And not ever, ever having seen one anywhere, I could probably serve as good evidence of their lack of usefulness!
I have seen them from time to time in several states on state highways and IIRC on interstates as well. Sometimes they are designated with signs saying “odometer test section” or the like, but they may be just markings on the pavement or shoulder; they are always on straight and fairly level stretches. In California I have seen markings at one mile intervals visible from the air in areas with signs saying the highway is patrolled by aircraft.
They are scattered around, I know of two that I frequently drive past, one in NH, one in MA. For places where people use snowtires that are often not the same diameter as their regular tires, it’s a good way to get an estimate of how off your speedometer/odometer will read.
Beyond that, I don’t see them as much more effective as the highway milage markers. But as kids we always used to check the car odometer against them. 
Odometers/speedometers are moving toward digital operation these days. Antilock brake tone wheels provide a signal. These can be calibrated with a fairly simple procedure.
A measured mile and a stop watch allows testing of speedometer as well as odometer. at 60 mph, it should take exactly 60 seconds. 55.4 seconds at 65 and 48 .0 seconds at 75.
It will also allow cops in a helicopter determine your speed quite easily.
But they’re aren’t marked in a way to be easily seen from the air.
There are roadway markings in some places which are designed for that purpose, however. They are not usually spaced a full mile apart, however. Normally the markings are spaced 1/8 or 1/4 mile apart. This system is called VASCAR (Visual Average Speed Computer and Recorder), and can be used by officers on the ground or in the air.
I guess I’m not understanding what the OP is asking then. I thought he was referring to the wide white lines painted on the pavement when he said “measured mile,” and differentiated it from the mile (and fractional mile) marker posts installed along the right of way.
VASCAR! That’s it. I knew there was a name for what I was talking about. Thanks for supplying it, QED.
Many years ago I drove a pickup that someone else had put over-sized tires on. Boy did it make a difference in speed. This was of particular concern since I daily traveled the Natchez Trace, which is a good place to get a ticket. 
Wow, I never thought it made that big a difference, so I calculated the following:
diameter, in 28 30 34
circumferance, in 87.9645943 94.24777961 106.8141502
Rps 1000 1000 1000
Rpm 60000 60000 60000
distance per minute, in 5277875.658 5654866.776 6408849.013
miles per hour 83.29980521 89.2497913 101.1497635
And was suprised to see the big difference this would make using what I guess are probably reasonable truck tire measurements.
Nope, different things.
What I’m talking about (and I think the OP as well) are special signs and measurements marking off a single mile very accurately on a stretch of road, independent of the milage markers usually seen along highways. The first sign will say “Start of Measured Mile” and the end sign will say “End of Measured Mile”.
These aren’t associated with VASCAR in any way.
I’ve never seen signs like that in Colorado.
I have seen the wheel dragged behind a DOT vehicle used to measure distance (been a while though [we used to use them, no more]). Perhaps they used the ‘measured mile’ to calibrate them (I sort of doubt it though). Thing of the past now. GPS/GIS can do it much better. Or at least more consistent.
GIS can provide a centerline down the road and measure any distance along curves. You can use aerial photos to derive the centerline, or each lane. Aerial photos will also tell the worker where that spot is. GPS can confirm it. Sort of depends on how often you take readings from the satellites, and how curvy the road is.
Could it be for your average Joe? 60mph in one minute would tell you if your speedometer is working correctly.
Or maybe it is for State Patrol. They can make sure their speedometers are working correctly. They double check their speedometers once a week so stuff doesn’t get thrown out on a technicality.