The
AADT from 2010 says that the highest 4-lane segment east of Toronto is a tie between Burnham Street and Highway 45 in Cobourg (that's now planned for completion of widening in 2016) and the bridge over the Trent River in Trenton (I believe the recent rehabilitation of that bridge included adding extra lanes that I assume they will open once the widened 401 reaches that far).
After that the next busiest stretch is from Highway 45 in Cobourg to the next interchange east. Between Trenton and Belleville is certainly not far behind though. I'd think that 4-laning will continue to Belleville and then to Kingston in small pieces - in the same manner it's been ongoing since the 1970s. Ultimately though, I can't seeing it go past the 416.
Yes I saw that stretch included in the Southern Highways document posted earlier. That's good, but that's still a pretty small stretch. And yes, when they rebuilt the Trent bridge they definitely widened it in the process.
I figured between Trenton and Belleville would be high on the list because those two cities are becoming increasingly intertwined, with the 401 being used a lot for commuter traffic as well, much the same reason why the highway was widened in Kingston (to deal with commuter + thru traffic).
And yes, I can't see it being needed east of the 416 either. In fact, east of Gananoque I can't really see much of a short term need, except for maybe widening through Belleville in order to deal with the quick couple of on/off ramps, and between Prescott and the 416, in order to deal with 416 traffic merging going westbound, and 416 traffic splitting going eastbound.
If I had to prioritize the widening, it would probably be (not including those that are already underway or are immediately impending):
1) Trenton on-route to just east of Belleville (on-route included in order to deal with the big hill you need to go up heading westbound, that really needs a 3rd truck lane).
2) Through Napanee and up the hill just east of Napanee.
3) Join the 6 lane sections between Cobourg and Trenton.
4) Through Belleville and from Prescott to the 416.
5) Join the 6 lane sections between Kingston and Napanee.
6) Join the 6 lane sections between Belleville and Napanee.
7) Extend 6 lanes east to Gananoque.
Note: I'm only talking about the eastern section of the 401 here, as that's the section that I drive most often. I'm only vaguely familiar with the details of the traffic issues on the west 401.