The Twelve Mile Circle examined freeways and motorways with the most lanes previously. That was a measurement of potential capacity. Would those massively-wide behemoths continue to reign supreme once someone posted actual traffic volumes? That wasn’t the case albeit with one notable exception.
Comparisons weren’t easy although Annual Average Daily Traffic (AADT) seemed to be a prevailing standard. In simplest terms, "it is the total volume of vehicle traffic of a highway or road for a year divided by 365 days." It can be a tedious exercise comparing values unless one enjoys wading through hundreds of pages of tables or spreadsheets — oftentimes not easily sortable — looking for the highest AADT. I can’t guarantee that I found the absolute highest traffic measurements in the world because I wasn’t that thorough, although I do believe I uncovered many of the more impressive values. Also I had to be careful to double-check that I was looking at AADT, a measurement for a specific point along a specific road, and not other measurements such as the complete traffic volume for the entire road.
Ontario Highway 401’s Busiest Segment
A segment of Ontario Highway 401 (a.k.a., King’s Highway, MacDonald-Cartier Freeway) definitely held the distinction of the highest traffic volume in North America, and possibly the world. I included that qualifier because it was the highest AADT I found anywhere on my own and because numerous sources with much greater knowledge of this subject yielded nothing higher. Maybe there could be a place in a highly-populated corner of Asia so I left the claim with a little asterisk.
The 401 was the notable exception mentioned earlier, appearing on the list for extreme lanes (20-ish) as well as AADT (400,000+). The maximum lanes occurred near Toronto Pearson International Airport while the traffic extreme happened a few kilometres farther east in what used to be the municipality of York, which became part of the City of Toronto in 1998.
Specifically, according to Ontario’s Ministry of Transportation, the 1.5 km segment of 401 between Highway 400 and Weston Road recorded an AADT of 403,300 vehicles in 2010. If that sounded bad, consider that it was closer to 450k in 2004 and sometimes peaked above 500k.
United States of America
Interstate 405’s Busiest Segment
The United States posted some pretty impressive vehicle totals, too. A table from the U.S. Department of Transportation’s Federal Highway Administration listed Most Traveled Urban Highways for the nation, specifically those with an AADT above 250,000. California utterly dominated the results with six of the top ten busiest roadways.
Top honors went to Interstate 405 in the Los Angeles-Long Beach-Santa Ana metropolitan area. I cross-referenced the FHWA table with data available from the California Department of Transportation’s Traffic Data Branch. The spot on I-405 with the highest AADT in the U.S. seemed to correspond to a segment between Rt. 22 and Seal Beach Blvd., in Seal Beach, California. It ran adjacent to the northern edge of the Naval Weapons Station there. About 377,000 vehicles passed through that brief corridor on an average day in 2008.
My little corner of the world, the Washington, DC metropolitan area, scored "only" 297k on Interstate 95; reaching 16th place. I kept that in mind for context as I explored other urban areas.
M25 Motorway’s Busiest Segment
I saw some impressive claims for the M25, the London Orbital motorway, although I couldn’t find a credible source for an AADT above 200,000. I did uncover a wonderful interactive map for areas throughout the UK and went off on a tangent exploring that for awhile. However I wasn’t about to click on every greatly-traveled road segment just to find the highest value. Rather, I punted and went with Wikipedia’s claim of 196,000 "recorded in 2003 between junctions 13 and 14 near London Heathrow Airport."
There were higher AADT values on continental Europe including 257k for the A4 motorway in Paris, France; 216k for the A 100 in Berlin, Germany; and 200k for the A23 in Vienna, Austria. None of those came anywhere near Canadian or American values so I didn’t pursue them further.
Sydney Harbour Bridge
I couldn’t determine a solid Australian candidate, and offer a challenge to the 12MC audience to help me find it. Sydney seemed to have the requisite population density so I focused there as a proxy. The highest value I found was 157,138 on the Sydney Harbour Bridge. That was all the way back in 2002 so values would have changed in the meantime. Interestingly, the same bridge had higher values a decade earlier (180k-ish). I learned that AADT dropped significantly on the bridge after the Sydney Harbour Tunnel opened in 1992, which seemed logical enough.
Auckland Southern Motorway’s Busiest Segment
I wasn’t searching specifically for New Zealand although I stumbled upon a claim and decided it was significant enough to feature. The segment of Auckland Southern Motorway between Khyber Pass Rd and Gillies Ave was generally considered to have an AADT of about 200,000. My examination of official numbers found a value considerably lower albeit fluctuations were common so it’s possible that the conventional wisdom on the Intertubes came from an earlier time period.