How many "Bridge Ices Before Road" signs are in the state of Michigan?
December 29, 2018 10:04 PM   Subscribe

Help me settle an argument. How many "Bridge Ices Before Road" signs are in Michigan? It seems like any time a road crosses another road, or a creek, river, lake, or stream, there are two signs. Sources vary greatly. Would love any estimates or napkin-math.
posted by bbqturtle to Travel & Transportation (7 answers total) 3 users marked this as a favorite
Well, I was all set to do some napkin math, but while gathering the necessary facts I stumbled on this MDOT page that claims Michigan has "10,754 roadway bridges". It isn't entirely clear what counts as a bridge (are overpasses bridges?), but an estimate of 20,000 signs seems reasonable on this basis.

Just for fun, though, here's my napkin. The same MDOT page states that Michigan has about 120,000 miles of paved road, of which over 80 percent (99,000 miles) is accounted for by trunk roads and county roads. Let's ignore the other 20 percent and make a simplifying assumption that the trunk/county roads form a grid with 1-mile spacing, like graph paper laid over the state. This isn't totally off the mark in many areas! But Michigan has about 97,000 square miles of area, which under the model above would mean 194,000 miles of road, twice as high as the actual amount. So let's just assume half the grid is missing (also not totally off the mark; there are some considerable areas without roads).

Michigan has just over 50,000 miles of river (source). Let's assume the paths of rivers are random and uncorrelated with the positions of roads (obviously not true, but we're working on a napkin, so...). According to the logic of Buffon's noodle, a given waterway should cross the 1-mile grid once per pi/4 miles on average, or once per pi/2 miles when you take into account that half our grid is missing. So you'd have about (50,000)/(pi/2) ≈ 30,000 crossings in the state, three times the number of bridges MDOT claims exist.

It's reassuring that both answers are within the same order of magnitude, at least! And it's not hard to believe that the true number of bridges is smaller than the model predicts; the roads are probably aligned in many areas to avoid a need for bridges.
posted by aws17576 at 1:35 AM on December 30 [14 favorites]

Here is a link to MDOT bridge info database where you can view info about the 5933 bridges in the MDOT system.

It has a column for SIGN_MESSAGE, but that appears to only be populated with certain directional signage.
posted by tronec at 3:37 AM on December 30

It isn't entirely clear what counts as a bridge (are overpasses bridges?), but an estimate of 20,000 signs seems reasonable on this basis.

In Michigan, overpasses are definitely bridges and get The Sign.
posted by LionIndex at 5:59 AM on December 30 [3 favorites]

The wildcard is box culverts for wide streams that aren’t big enough for bridges. They aren’t technically bridges but they’d have the same early icing effect.
posted by hwyengr at 7:32 AM on December 30 [2 favorites]

Yeah, my concern on the miles of Rivers is the box coverts that are everywhere and seem to also have signs for the bridge, but don't have a name per say.
posted by bbqturtle at 7:57 AM on December 30

I'm willing to bet that the venn diagram of "What MDOT calls 'roadway bridges'" and "What MDOT puts 'Bridge Ices Before Road' signs on" is very nearly a circle -- if a culvert or ditch or whatever is wide enough to justify a BIBR sign, then apparently MDOT considers it a bridge. So 21,508 is probably closer than any napkin math would get you.
posted by Etrigan at 11:41 AM on December 30 [2 favorites]

Not all the signs have been changed either, the older ones that say, "bridge may be icy" are still on a lot of the bridges and overpasses around West Michigan.
posted by chocolatetiara at 3:37 PM on December 30 [1 favorite]

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