The desired answer will describe the location of all the 420 mile
markers in the continental United States.  By this I mean all points
on US highways where the mile marker is mile 420.

I think (but am not certain) that there is one of these near Couer
d'Alene, ID.  I'd like to know if this is true, and if there are any
others in the US, where they are located.

Hello Joel,

Yes, there is a mile marker 420 near Couer d'Alene. It's south of the
city on U.S. 95 between Rockford Bay and Mica Bay on Lake Coeur
d'Alene in Kootenai County. This information comes from Zach
Maillard's Idaho Highways' Homepage at
(http://www.uidaho.edu/~mail9492/). A picture of a mile marker on U.S.
95 is available at the Idaho Panhandle National Forests Web site:

<http://www.fs.fed.us/ipnf/eco/projects/mcarthurlake/>

As for other mile marker 420s, I went to the International House of
Zzyzx (www.ihoz.com) to look for all of the interstate highways that
run for at least 420 miles in a given state. I found the following:

I-5 California
I-10 Texas
I-20 Texas
I-25 New Mexico
I-35 Texas
I-40 Tennessee
I-40 North Carolina
I-70 Colorado
I-70 Kansas
I-75 Florida
I-80 Nebraska
I-90 Montana

You should be able to find where along the highway a particular mile
marker is by looking at a road map. For example, mile 420 on I-10 in
Texas is between Exit 404 (Sonora) and Exit 437 (Roosevelt). An
exception is I-5, since California is just now putting on exit
numbers. According to CalDOT at
(http://www.dot.ca.gov/hq/traffops/signtech/calnexus/reports/fivenorth.htm),
Mile 420 should be between Route 140 East and Stuhr Road near Newman.

As for U.S. highways in other states, U.S. 12 in Washington State
should have a Mile 420 at 10.81 miles west of the Idaho state line,
according to Mark Bozanich at
(http://www.angelfire.com/wa2/hwysofwastate/us012.html).

Presumably, Montana has several Mile 420s on U.S. highways, but I did
not find a comprehensive source of information on the topic.

Good luck with your quest.

Search strategy:

Idaho highways
<://www.google.ca/search?q=Idaho+highways&ie=UTF8&oe=UTF8&hl=en&meta=>

Idaho mile markers
<://www.google.ca/search?hl=en&ie=UTF8&oe=UTF8&q=Idaho+mile+markers&meta=>

U.S. highways
<://www.google.ca/search?q=U.S.+highways&ie=UTF8&oe=UTF8&hl=en&meta=>

Thanks!  The answerer's answer, combined with the comments posted on
this question, have given me all the information I'll need to, ah, see
all the 420 mile markers. Thanks again! :)

Even if this was of interest to someone else, I doubt that they would
have published the information.

There appears to be one in NC, if their exit numbers equal mile
numbers:
http://www.interstatehiwayguide.org/carolina/nc_i-40E.htm#exit

Wyoming has a 419.23 and a 425.42:
http://www.aaroads.com/wyoming/us287-1.html

And there is one in Louisiana (.pdf file):
http://www.gomdot.com/cetrp/annex_g_bridge_damage_over_water.pdf

There also appears to be one in Kansas with a speed trap:
http://www.beartraps.com/i-70.asp

There probably is one near Coeur d'Alene.

Assuming you want to do what I think you want to do, I think you're
very silly. In a good way, though.

I'm with bookface-ga on this one...but I think I'd be more likely to
drop what I'm doing and go with you!  Good luck, and I hope the photos
turn out.  Heh.

Sounds like the Kansas State Troopers are looking for more than just speeders.

Let's see how long it takes till somebody posts the question "What
happened to all the 420 mile markers in the continental US?"

Sure people say they're gonna visit all of them :)

What are the odds that you'll be too... um... tired to finish the job?

I dare you to get your picture taken in front of all of the 4.20 mile
markers next (my advice - start off with Rhode Island - there can't be
that many markers).

Actually, if you're too lazy to go to all of them yourself, you could
set up a website similar to www.confluence.org and get people to pot
pictures of themselves next to the markers.

Calebu2

".....get people to pot pictures of themselves ..."

Your slip is showing ...

--S. Freud

Why 420?

Responding to bode-ga's "Why 420?":

http://www.wired.com/news/culture/0,1284,51986,00.html

This should explain everything.

How did this experiment go? Was it sucessful. I'm interested in a progress report.

The experiment has not yet begun.  Next summer is probably when it
will happen.  If it does, I will be sure to post picture URLs on this
google-answer. :)

.. I hope they don't start 'archiving' the answers after a while.