  Let $T$ be an algebraic torus over an algebraically closed field, let $X$ be
a smooth closed subvariety of a $T$-toric variety such that $U = X \cap T$ is
not empty, and let $\mathscr{L}(X)$ be the arc scheme of $X$. We define a
tropicalization map on $\mathscr{L}(X) \setminus \mathscr{L}(X \setminus U)$,
the set of arcs of $X$ that do not factor through $X \setminus U$. We show that
each fiber of this tropicalization map is a constructible subset of
$\mathscr{L}(X)$ and therefore has a motivic volume. We prove that if $U$ has a
compactification with simple normal crossing boundary, then the generating
function for these motivic volumes is rational, and we express this rational
function in terms of certain lattice maps constructed in Hacking, Keel, and
Tevelev's theory of geometric tropicalization. We explain how this result, in
particular, gives a formula for Denef and Loeser's motivic zeta function of a
polynomial. To further understand this formula, we also determine precisely
which lattice maps arise in the construction of geometric tropicalization.
