What is the significance of the season in the poet?
There are several possible levels of significance to the setting of the poem in the autumn season. On the surface, it could simply be an easy way to note that one road has not been traveled as much as the other - the fallen leaves are not disturbed as they lie on the road.
On a deeper level, perhaps the speaker is noting that his life's travels have progressed to a junction - a point at which he needs to make a decision that will significantly impact the form of the rest of his life. This is a consideration that needs to be seriously examined, and the speaker recognizes that, once his decision is made, "I doubted if I should ever come back" - the effects of the decision would probably be permanent.