The southern terminus of US 45 and SR 17 is at their intersection with US 98 and unsigned SR 42 in Mobile. US 45 and SR 17 assume a northwestward trajectory as they leave Mobile heading towards the Mississippi state line. Approximately north of an interchange with Interstate 65 (I-65) northwest of downtown Mobile, the routes quickly enter rural areas of Mobile County via a two-lane highway. Near the small community of Deer Park in southern Washington County, SR 17 diverts from US 45 and becomes a signed route.