I saw a tumbleweed get hit by a truck on the freeway!
They are only a barrier to the first tumbleweeds to arrive there.
It's very like some depictions of a zombie attack, and for a very good reason; tumbleweeds are no longer alive.
Think about it; desiccated and uprooted, they are no longer alive, except that in among the tangled branches, there may be seeds, which are the purpose of its journeying.
(Time to cue up the Sons of the Pioneers.)