Mile Markers

I just heard the segment on today’s radio show where the brothers suggested that Montana has put it’s mile markers closer together than one mile in order to make drivers who check their speedometers with a stop watch think they are going faster than they actually are. (Devide 3600 by the number of seconds between markers to determine MPH)

Since federal funds were used to build the interstates, and most state highways, there is no way the government doesn’t know EXACTLY how many miles we’ve all paid for. I can remember “chaining in” those markers as a surveyor on a section of road when I worked for my state’s highway dept in the ‘70s. Believe me, they were 5280’ apart. I doubt they’ve moved.

This post has been moved to the new Car Talk Discussion Area, by a Car Talk Lackey. The original poster is MG_McAnick.

With faster speed limits, I bet they aren’t silly enough to put the markers closer. Plus, they would have to buy more markers, something to put them on, more cement. Montana is kind of big. There’d be Helena to pay.

More likely the person who brought this up has oversize tires, changing the odometer readings.

My thoughtd exactly.

While an individual marker may be slightly out of place overall they are very accurate. Using your odometer is usually not all that accurate. Different tyres and the usual ~5% optimistic readings make you think you are really going a little faster than you really are.

It is not hard to test this out. Get in your car and bring along a good GPS device. Measure a long (50 miles +) distance with your odometer and with the GPS and with the mile markers. See which two are closest.

All of the GPS receivers I am familiar with have speed displays which you can match to your speedometer as you drive along. There is no need for averaging over long distances.

Speedometer accuracy varies from car to car. In most, the errors are less than 1 mph and are due the fact that different models of tires with same nominal size are not necessarily the same physical size. In a few cars, the speedometers are significantly optimistic. I have never encountered a speedometer that read below the car’s actual speed. (Of course, this assumes that the car is eqipped with tires of the correct nominal size.)

There are several things which cause mile markers to not be exact:

  1. Occasionally the exact point where a mile marker belongs is within an incompatible feature (such as an intersection). If this is the case, the marker is placed as close as it is safe to place it.

  2. The mile markers will not be a full mile apart where one marker is at a state line.

  3. Where a route’s roadway has been changed, the markers at one end of the change might be up to several miles off (until the state has time to replant the markers beyond the change).

  4. Where several routes merge together, the markers show the miles of the most important route. The other routes pick up their accurate mileage when they diverge from the main route. The markers on the less important routes are not a full mile apart at the merging points.

Just remeniscing here, when the Interstate highways were built in the 1950’s and 1960’s the speed limit was 70. The speed limit was lowered to 55 by Nixon with the Arab Oil Embargo in the 1970’s.

US highways and all major arteries were usually 65, with a reflective surface on the sign that changed it to 55 when your headlights illuminated it. Country roads, including gravel roads were considered 65 MPH unless otherwise posted

US highways were the predecessor of the Interstate highways, and they were started in the 1930’s Lanes were about 12 feet wide with no paved shoulder. We lived on US 12 that was the main artery from Chicago to Detroit in the 1950’s. The speed limit was 65, and people would pass anyone driving that slow, even the Greyhound busses were going about 80 or 90 past our house. Of course, school busses were stopping, too, which made for in interesting mix.

Mile marker 1 will be one mile from the state line. the last mile marker will be one mile from the next to last mile marker, the difference above the last mile marker in the state will be shorter, until you cross the state line and mile marker 1 comes around again, making it seem to be a longer distance between mile markers.