うえに

この道は狭い ___ 、交通量が多い。

The correct answer is 「うえに」. This pattern, Verb-plain/Noun+である/Adjective + 「うえに」, means "in addition to..." or "on top of that...", connecting two facts or qualities where the second adds to the first, often implying an accumulation. The road is narrow, and *additionally*, it has heavy traffic. 「反面」 means "on the other hand..." and expresses two contrasting aspects of the *same thing*. While narrowness and traffic are related, they are not necessarily contrasting aspects of the road itself in a trade-off sense. 「ばかりか」 means "not only... but also..." and often implies something surprising or of a greater degree, which is not strongly present here. 「かわりに」 means "instead of..." or "in exchange for...", implying a substitution or trade-off, which does not fit this context.

正解は「うえに」です。普通形に続き、「～であることに加えて」という意味で、二つの事柄が重なることを表します。道が狭いだけでなく、交通量も多いという事実を追加するので適切です。「反面」は同じ物事の二つの異なる側面を表しますが、この文脈では単に二つの事実の追加です。「ばかりか」はさらに驚くべき事柄が続くときに使い、ニュアンスが異なります。「かわりに」は代替や引き換えの意味で、文脈に合いません。