Are the words treacherous and dangerous interchangeable?

They have the same denotation but different connotations. Something that's dangerous, you know it's dangerous. Like motorcycling or smoking or walking through bad neighborhoods late at night. Something that's treacherous, you don't know it's dangerous. Like a false friend or a snake that hides under your pillow. I would guess that 'treacherous' comes from the same root as 'treasonous'.

Treacherous implies that the danger comes in unexpected ways, times, and directions, or is greater than it appears.