I ran across this question by accident on Yahoo, but the responses seemed to be largely composed of WAGs and potential misinformation. What better place to bring the question, then, but to the Doper masses?
I did a quick search, but didn’t see this answered elsewhere. Forgive me if this has already been done.
I don’t know the undoubted answer, but the widesty I do know of is this: in the vicinity of Exit 9, the New Jersey Turnpike has 12 lanes, not counting exit and entrance ramps. There are three lanes each for Cars Only and Cars Trucks + Buses, and there are North-Bound and South-Bound for each.
Avenido 9 de Julio, in Buenos Aires, is always mentioned as the World’s largest street, (especially by Porteños :)), but it’s not a freeway.
At its widest point, I want to say that, the 401 in Toronto has 7 or 8 lanes in each direction
Toronto Metro has been mentioned as having the world’s widest superhighway (the 401? I’m not sure if it was), but I have no details.
I’d suggest that we may want to refine our definition a bit here: things such as toll plazas, multiple exit lanes, and such may not be valid answers to the question asked, or may only count as specialized answers. There may be two questions:
What road has the most “through” lanes (as opposed to lanes turning into exit ramps, widenings for toll plazas, etc.)? For example, the criterion might be that the road must be that wide for 3 miles (5 KM) before losing lanes to qualify.
What superhighway is at some given point the widest? In this case, toll plazas, multiple lanes for exits, etc., would count.
Yes, there are a number of places where it’s 3, 4, centre barrier, 4, 3. Then you add ramps, transfer lanes, and merge lanes. The largest ‘instantaneous’ number of lanes at any point that I can think of is 20 (counting them in my mind), south of the airport, at the express/collector crossover just east of Etovicoke Creek.
But the express/collector system is almost continuous from the 403/410 interchange in Mississauga to Whites Road in Pickering, with only that one gap east of the airport where the highway reduces to 5 lanes in each direction. This is at least 30 km that is a minimum of 3, 3, 3, 3. So I suspect that the 401 wins in total ‘area’ (lane-miles or lane-km).
My recollection is that the stretch of the combined I-75/I-85 that runs through Atlanta has a very large number of lanes in each direction. I have a vivid nightmare I relive from my first time through there, during thunderstorms dropping a drenching downpour, late in the evening rush hour, while crazy Georgians were speeding along at upwards of 80 mph.
Did I mention the fact that travelling through on either interstate, you mush shift lanes because I-75 and I-85 CROSS each other at that point (that is, I-85 comes in from the north of Atlanta East of I-75, and exits Atlanta West of I-75)??! Not even the downtown mixing bowl in LA comes close for hairy-scary rides.
Near Toronto Airport, the 401 actually reaches 9 lanes in either direction, for a total of 18.
Yep - I was going to mention the 401 near Toronto as well. It’s wacky!
OK, my only knowledge of these is through Google Earth, but from what I can see the Toronto case never gets beyond 4-5 lanes on any one undivided stretch, whereas DSYoungEsq’s Atlanta example really does have eight or more lanes all side-by-side. (Happy to be corrected!)
I’m not sure what you mean by this - I’ve driven on the 401 and there really are 18 lanes as you’re coming up and around the Toronto airport - 8 go straight through, and 10 are for entering/exiting the freeway (that is, you enter on one and then have another 4 that you can move over into in each direction.)
I’m not sure why that wouldn’t show up on GoogleEarth but I’ve seen the monstrosity with my own eyes and was glad that my rental was a Mustang with a bit of get up and go. 
K - I just looked at GoogleEarth and the 401 is 18 lanes wide (9 in each direction) coming into the city by the airport.
It continues that way for quite a distance too.
I’m pretty sure we’re talking about the same section of road (east-west, just south of the airport?) If I’m wrong I apologise, but it looks to me like paired 3/4/5 lane highways, with a shoulder & median separating them, not a nine-lane free-for-all. If this isn’t what it’s like on the ground, then what is it?
Ah - I was wondering if I was misunderstanding. It’s not a 9-lane free for all. If you get into the middle 8 lanes you’re sort of stuck there until you get to London (or somewhere) - there are 5 lanes on eitherside that are the free for all. And at some point where you enter the centre 8 lanes it’s a 9 lane free for all, but it doesn’t last that long.
Anyway - it’s 18 lanes wide. It’s a frikin’ monstrosity and at somepoint you can negotiate your way in and out of all of the lanes, but not near the toronto airports.
Ontario, I hope! Either that, or somebody had better widen the M4 damn quick 
Well, I could link you to an Amazon promo for Harry Harrison’s A Transatlantic Tunnel, Hurrah!, or tell you that the Canadians are serious about Her Majesty being their Queen too, and have a top-secret road/bridge/tunnel she can use to come to her Canadian realm (you should see the drive-on-left/drive-on-right crossover! ;)). Or we could consider that it’s probably London, Ontario.
-Poly (who grew up 13 miles from the second-largest Copenhagen in the world)
Well, I grew up near the second-largest Ipswich in the world, which means Queensland is growing too damn fast. (There’s still Ipswich, South Dakota to keep our egos intact 
)
You’re pretty correct for most of the 401 through Toronto. The only part not like this in the Metro area is what we used to call “the Martin Grove bottleneck,” which is just east of the airport. It’s indeed a free-for-all.
Anyway, it may be confusing to those who haven’t seen or driven it before. Essentially, the 401 is like the New York subway: there are express lanes, in the middle; and collector lanes, on the sides. Just as the express and local trains make up the subway, so do Express and Collectors make up the 401. Collectors often handle local traffic–those cars going one or two or three exits, say–while Express handles cars going farther; or indeed, just through the Greater Toronto Area. But note that regardless of how far you’re going or what set of lanes you want to travel in, you will always enter and exit the freeway from Collectors.
The express/local analogy is the theory. In practice, it differs. You can, if you like, get all the way across the GTA in Collectors; or you can get on Collectors, get over into Express, and exit Express to collectors at the first (or second or third or whatever) opportunity. This is sometimes necessary if you expect to get anywhere–an accident in Collectors that forces Collectors to come to a complete halt does not stop Express (though it often slows it down as people slow to look). And vice-versa.
To Torontonians and others who have, shall we say, experienced the 401 through the GTA, there is no difference–in other words, the 401 is not “the lanes in the middle,” and neither is Collectors just on- and off-ramps The speed limit is the same for Express and Collectors, highway markers identifying the highway as 401 are in Collectors, and you can get through the GTA in Collectors. Looked at this way, the 401 is just as wide as other posters have said: adding Express and Collectors eastbound and Express and Collectors westbound, you get something like 16 or 18 lanes wide at its widest.
Spoons
(who drove the 401 across the GTA daily on his way to work when he lived in Toronto)
There’s a reason I call this highway the Evil Death Highway of Doom. 
If you break down in the express lanes, you are essentially trapped; you cannot reach the shoulders of the highway, because the collectors are in the way. Transfers between the collectors and the express lanes are fairly far apart.
If something, say a flipped truck spilling its load (this happens at least once a year) blocks the express, traffic behind will be trapped, unable to escape to the collectors. And the same holds for the collectors.
There are some bad design decisions on the highway as well, the unexpandable 401/427 interchange among them. On the way home from work, the bus I ride regularly gets trapped in a configuration where seven lanes merge down to four in about a kilometre (eastbound 401 to southbound 427, if you must know). Guaranteed traffic jam.
Of course, Wikipedia has an article about Highway 401, which gives a detailed lane configuration for the whole length.
[hijack] So when I visit Toronto on Business they have me picked up by a company called GTA Limo. I always expect a guy that looks like Snake Jailbird to drive up in a limo where the igniton lock has been punched, and we get chased by the cops at high speed on the way to the hotel.
In the US GTA = Grand Theft Auto[/hijack]
