For the convenience of travelers, there are many rest areas along the highway.

Literal

Travelers [possessive-の] convenience [object-を] seek highway-along [location-に] many rest-areas [subject-が] exist.

便宜をはかって (for the convenience of) uses 図る (to plan/seek) — 'having planned for convenience.' 高速道路沿い (along the highway) uses 沿い (along) as a suffix. A descriptive, informational sentence.