15 minutes by rowing boat anywhere in lake Huron near the Sault border.
But this guy was trying to leave.
On top of that there is a much easier way to get a bomb in to the US, buy it in bits and pieces in the US, assemble it there. No point in taking it across the border.
Security theater does not equal actual security. If someone actually wanted to drive a bomb across the border, I'm sure they'd find a way.