"I go slowly on a narrow road."
When we put は in this case, there must be some sentences before this sentence: such as 太い道は速く、細い道はゆっくりと進みます。(We go fast on a wide road, but we go slowly on a narrow road.) は can be used when you want to compare 2 things, and we sometimes use は for the latter. For example, 私は彼女にプレゼントをあげるけど、彼はいいだろう。(I give her a present, but not to him.)
The use of は here implies that the speaker is comparing narrow roads to something else. For example, as "I go slowly on narrow roads, but not on wide roads"
I'm no expert, but I think this is a bit misleading for beginners without more context or explanation about why は is being used.