AMP: 18 Location: S-421 and US-212, Carbon Cty No reassurance shield as 421 begins. AMP: 8 Location: S-421 at the Carbon County Line, Stillwater Cty Reassurance shield about halfway down the route as we enter Stillwater County. AMP: 0 Location: S-421 and MT-78, Carbon Cty A few miles later, we’ll approach Highway 78. AMP: 0 Location: S-421 and MT-78, … Continue reading Montana Secondary Highway 421, US-212 to MT-78