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FM signals follow line-of-sight, and do not reflect off the ionosphere. So curvature of the earth impedes the signals from going further than that. On fairly level ground, that means about 75-100 miles from a standard height tower to a receive on the ground. FM frequencies are in the band between TV VHF channels 6 and 7, so distance of propagation would be about the same. When you drive your car into a steep valley, you will often lose your signal, because you are out of line of sight. Tall urban buildings can have the same effect, cutting out weak signals in large cities. In the mountains, signals will never make it over the pass, no matter how close it is.
However, they are subject to "skip" in certain weather conditions, and occasionally, you can pick up a station over 1.000 miles away, whose signal is bouncing off an atmospheric interface, something like the mirage of the highway that follows a line of sight reflecting off hot air above the road. This usually occurs on hot summer days. I was once driving in western South Dakota, and picked up dozens of stations in the Carolinas, because there just happened to be a heat inversion between those two points to reflect the signal.
That sounds like ionospheric (sporadic E or "Es") and not tropospheric ducting. Tropospheric ducting is somewhat rare in western South Dakota due to the aridity of the climate, but Es is common throughout the U.S. in May, June, and July.
FM signals don't seem to travel too far but ...
AM that's a different story.
From Toronto (even sometimes Montreal) at night I would dial in
WMAQ from Chicago, they must have a really powerful transmitter.
Also from Montreal I would also listen to NYC and New Jersey AM talk radio stations
(again, I could only get at night).
Here in Charleston, SC, the FM stations vary. The weaker ones normally come in for around 30-40 miles of the metro, while the stronger FMs (most of which with 100,000 watts) can be heard at least 80 miles even on a normal day.
The stronger Charleston FMs can be heard as far as the Myrtle Beach area with a good antenna. Any skip brings the signals into NC.
They can be heard as far south as Hilton Head, and some can be heard nearly to Columbia. Much of their signal is put out over the ocean, so it would go out even farther otherwise.
Location: Prince Georges County, MD (formerly Long Island, NY)
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About 10-12 years ago, we were driving down I-95 through Harve de Grace, MD in the middle of the night (3AMish?) and picked up 1010AM from New York City. It must've been a fluke because we tried again on subsequent trips with no avail.
I live on the east side of Athens, GA these days, and I'm about as far east as you can get clear radio signals from Atlanta on a normal day.
Atlanta area*
I-75 NB: Calhoun, GA
I-575 NB: Jasper, GA
U.S. 19 NB: Dahlonega, GA
I-985/U.S. 23 NB: Lula, GA
I-85 NB: Commerce, GA
U.S. 29 NB: Athens, GA
U.S. 78 EB: Athens, GA
U.S. 278 EB: Lake Oconee
I-20 EB: Lake Oconee
U.S. 23 SB: Lake Juliette
I-75 SB: Forsyth, GA
U.S. 19 SB: Thomaston, GA
I-185 SB: Pine Mountain, GA
I-85 SB: LaGrange, GA
I-20 WB: Heflin, AL
U.S. 78 WB: Heflin, AL
U.S. 278 WB: Alabama state line
Pittsburgh area*
I-79 NB: Grove City, PA
U.S. 119 NB: Indiana, PA
U.S. 422 EB: Indiana, PA
U.S. 22 EB: Blairsville, PA
U.S. 30 EB: Ligonier, PA
I-70/I-76 (Pennsylvania Turnpike) EB: Donegal, PA
U.S. 119 SB: Uniontown, PA
U.S. 40 EB: Uniontown, PA
I-79 SB: West Virginia state line
I-70 WB: Wheeling, WV
U.S. 22 WB: Steubenville, OH
U.S. 30 WB: East Liverpool, OH
I-76 (Pennsylvania Turnpike) WB: Ohio state line
I-376 WB: New Castle, PA
*: Assuming normal conditions and maximum permissible transmitter power.
Southern California: LA stations are pretty much heard until you get past Valencia in the north. Around Dana Point/SJC area is where they begin to fade in the South, but once you get to Oceanside you'll hear San Diego stations.
Portland: About halfway to the coast in the west they will begin to fade, around Longview in the North, and the stations may change when you hit Salem in the South.
Houston: I've heard Houston stations as far out as Galveston and Bolivar, around where they fade is uncertain, but I do know they fade before you get to Beaumont.
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