Commonwealth Avenue, formerly known as Don Mariano Marcos Avenue, is a 12.4-kilometer (7.7 mi) highway located in Quezon City, Philippines. It spans six to eighteen lanes, making it the widest road in the country. The avenue is one of the major roads in Metro Manila and is designated as part of Radial Road 7 (R-7) of the older Manila arterial road system and National Route 170 (N170) of the Philippine highway network.

Commonwealth Avenue starts at Elliptical Road, which encircles the Quezon Memorial Circle. It passes through the areas of Philcoa, Tandang Sora, Balara, Batasan Hills, and Fairview and ends at Quirino Highway in the Novaliches area.

Being located in Quezon City, which is among several cities in Metro Manila with a high incidence of road accidents, the avenue has a high accident rate, particularly due to overspeeding, earning it the nickname "Killer Highway". A speed limit of 60 kilometers per hour (37 mph) is being enforced to reduce accidents on the avenue.

Commonwealth Avenue follows a curving route from Elliptical Road to Quirino Highway. It is divided into two portions: the eighteen-lane main segment, formerly Don Mariano Marcos Avenue, and the six-to-eight-lane Fairview Avenue. The Fairview Rotonda, a roundabout at the intersection with Doña Carmen Street at the barangay boundary of Commonwealth and Fairview, marks the division between the two segments.

The main segment, formerly Don Mariano Marcos Avenue (after Mariano Marcos, the father of former Philippine President Ferdinand Marcos), stretches south of Fairview Rotonda. It features partial access control with interchanges and U-turn slots replacing at-grade intersections and pedestrian crossings placed on overpasses (footbridges). This segment has 18 lanes, with nine lanes per direction, excluding dedicated lanes for motorcycles, buses, jeepneys, and bicycles. An exclusive motorcycle lane, implemented in March 2023, restricts motorcycles to the third lane except when entering or exiting the highway.

On the other hand, Fairview Avenue, north of Fairview Rotonda, has 6 to 8 lanes, and most intersections are at-grade, usually with traffic lights.

Being located in Quezon City, which has a high number of road accidents, Commonwealth Avenue has a high incidence of accidents along with Quezon Avenue. The number of accidents on the avenue has lent it its nickname, the "Killer Highway".

The outermost lanes of Commonwealth Avenue from Quezon Memorial Circle to Doña Carmen Avenue are designated as bi-directional bike lanes with plastic barriers and 0.6-meter (2.0 ft) buffer zones on both sides as part of Quezon City's bike lane network. A portion of the bike lane also uses concrete plant boxes instead of plastic barriers, with the city planning to replace plastic barriers with plant boxes.

Before the establishment of the city bike lane network during the COVID-19 pandemic, a bi-directional grade-separated bike lane was established by the MMDA in 2012 along a 2.92-kilometer (1.81 mi) segment of the highway from University Avenue to Tandang Sora.

The Metro Rail Transit Line 7 (MRT 7), which will connect with the Metro Rail Transit Line 3 at North Triangle Common Station, began construction in 2016. Most of its alignment will follow the center island of Commonwealth Avenue up to Regalado Highway.

The avenue's origins can be traced back to a segment of a road that connected the proposed site of the National Capitol in what is now Elliptical Road to the University of the Philippines Diliman campus and comprises what is now University Avenue. The original 1941 Frost Plan envisioned a road network connecting the proposed Capitol to the proposed Philippine Military Academy (PMA) through barrio Balara. However, by 1949, the revised Frost Plan shifted focus to constructing a road directly connecting Quezon Memorial Circle, which replaced the original Capitol site that was scrapped after World War II, to the new National Government Center (location of the present-day Batasang Pambansa Complex) at Constitution Hill, which replaced the proposed PMA site.

Originally named Don Mariano Marcos Avenue, the avenue was eventually constructed in the late 1960s as a two-lane highway. Quezon City was then the capital of the Philippines, and embassies were to be put up on the stretch of highway. Because the country's capital was moved back to Manila in 1976, other establishments were put up instead. Don Mariano Marcos Avenue was later renamed into two parts, Commonwealth Avenue and Quezon Avenue. Later, Commonwealth Avenue regained the eight-lane Fairview Avenue, which used to end near Jordan Plains Subdivision in Novaliches.

In the 1980s, the road was widened into a six-lane highway. During the late 1990s and early 2000s, the avenue was prone to heavy traffic and accidents due to increasing public utility vehicles plying the highway and sidewalk vendors crowding onto the road. In the late 2000s, the Metropolitan Manila Development Authority (MMDA) cleared the sidewalk vendors, especially in the Tandang Sora area, which was prone to heavy rush hour traffic. Fairview Avenue uses stoplights and center island splittings at its intersections, while Don Mariano Marcos Avenue uses interchanges at its intersections.

The avenue is 18 lanes at its widest. It is the widest road in the Philippines, beating the old record set by EDSA.

On October 1, 2009, Quezon City Mayor Feliciano Belmonte Jr. announced his 15-year-old plan to link Commonwealth Avenue and Quirino Highway at a cost of ₱20 million to ₱140 million, adjacent to the Zabarte Road. The project was completed in May 2011 and is already operational.

In May 2011, a 60-kilometer-per-hour (37 mph) speed limit was implemented on Commonwealth Avenue following the death of Lourdes Estella-Simbulan, a journalist, in a road accident on the avenue. During the first week of its implementation, 120 violators were apprehended after speeds of over 60 kilometers per hour (37 mph) were recorded through speed guns.

During the 16th Congress, Representatives Miro Quimbo (Marikina–2nd) and Erlinda Santiago (SAGIP Partylist) filed separate House bills to rename the avenue into Eraño G. Manalo Avenue, after Eraño Manalo, the second Executive Minister of the Iglesia ni Cristo, which has a central temple along the avenue. Meanwhile, the Makabayan bloc wanted to rename the avenue after former Filipino senator Lorenzo Tañada.

The entire route is located in Quezon City. The list includes the section of the avenue named "Fairview Avenue". Kilometers are measured from Rizal Park, the Kilometer Zero. 

km	mi	Destinations	Notes
12.491	7.762	N170 (Elliptical Road)	Quezon Memorial Circle, southern terminus. Continues to Manila as Quezon Avenue.
		Masaya Street	Accessible from southbound lanes via U-turn slot. Access to U.P. Village, Teachers Village and Kalayaan Avenue.
12.879– 13.051	8.003– 8.110	N175 (University Avenue)	Accessible from southbound lanes via U-turn slot. Access to University of the Philippines Diliman.
		Central Avenue	Accessible from northbound lanes via U-turn slot. Access to New Era University and Iglesia Ni Cristo Central Temple
		Tandang Sora Avenue	Right turns only. Flyover closed due to MRT Line 7 construction.
		N129 (Luzon Avenue/C-5 Road)	Access to Luzon Avenue residential areas.
		Zuzuarregui Street	Southern segment has access to Broadcast City
		Holy Spirit Street, Capitol Homes Drive	Holy Spirit Street intersection accessible from northbound lanes via U-turn slot. Capitol Homes Drive intersection accessible from southbound lanes via U-turn slot. Access to Ever Gotesco Commonwealth.
		IBP Road	Accessible from southbound lanes via U-turn slot. Access to Batasang Pambansa on both ends. Access to Payatas Road and the municipality of Rodriguez (Montalban) at the northern end.
20.382– 20.682	12.665– 12.851	Doña Carmen Street	Roundabout. Access to Don Jose Heights. Name change from Commonwealth Avenue to Fairview Avenue. Change from 18-lane road to 6-lane road.
22.041	13.696	Regalado Avenue	Traffic light intersection
23.242	14.442	Regalado Highway	Traffic light intersection. No left turns from southbound lanes. Access to SM City Fairview.
		Mindanao Avenue	Traffic light intersection
25.203	15.660	N127 (Quirino Highway)	Northern terminus.
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Incomplete access

14°39′22″N 121°3′22″E / 14.65611°N 121.05611°E / 14.65611; 121.05611