It passes Queluz in the edge of Sintra, then through the Amadora, Odivelas and Loures municipalities before ending after 35 kilometres (22 mi) at an intersection with the country's principal northbound highway, the A1.

Despite running for its entire length within 20 kilometres (12 mi) of central Lisbon, much of the route of the A9 is semi-urban or rural, incorporating mountainous regions that necessitated the inclusion of 15 viaducts and two substantial tunnels in its construction.

There was an idea to extend the westerly portion of the ring highway southwards across an additional bridge, but extending the A9 in this way appears not to have evolved beyond the status of an ambitious line on the map, and a second major highway bridge crossing the Tagus to the east of Lisbon has been constructed in the interim.

As of 2012[update] the charge for a passenger car to travel the full 35 kilometres (22 mi) from one end of the road to the other is €2.95.

In the middle of the afternoon on 24 January 2010, near Belas, next to the junction with the newly opened the A16 a violent landslip deposited a large quantity of rocks and mud on the road.