A single lane roundabout has a surprisingly high capacity, even without right turn and sometimes T roundabout bypasses. With at grade bicycle crossings they can have up to 1500 motor vehicles per hour, for a total of 36 thousand per day. Without at grade bicycle crossings, this can happen either with grade separation or a lack of any need to put a bike crossing near the roundabout, or at least not on a crossing, it can go up to 1750 per day, for a total of 42 thousand. For a single lane roundabout, I'm cool with that.
So what? Don't our roads have too much volume for that?
Actually no for the most part. Yes there are some roads where it would be ill advised to narrow it to a single lane or use a single lane roundabout, but a large number of roads can be narrowed successfully.
Some intersections do not have the space for a single lane roundabout without obtaining private property. So how to deal with that? Well, in many cases, a mini roundabout with similarly shaped bicycle paths around it as with normal single lane (non annular), but with the circle in the middle traversable, low and small and the roundabout being much smaller often works. It has the expectation that you can go over it if you are a truck, bus, pulling a trailer or for some reason need the central island to go anywhere, but most traffic is comprised of private cars or smaller vans. And in many cases, the buses can be re routed away from mini roundabout locations, not all, but some, and trucks can be regulated, perhaps no longer than 10 metres and no heavier than 7.5 tonnes (7500 kg), and school buses would be mostly a thing of the past because all the kids are on their bicycles, walking or taking the city bus, maybe creating exceptions for winter, and smaller minibuses 9 metres long perhaps for those with disabilities where it would be a challenge to use other means.
Parking on these roads, especially collector ones, is often not needed, especially given that on the arterials where there are destinations directly off the roads without a parking lot for it's own use tend to be in areas of the city where it's on the grid system, and in areas where there is even more potential for cycling, walking and transit than usual. Collector roads might have some parking left if there are homes and businesses without parking lots directly next to them, but can only be considered after cycling and walking both have safe and efficient places to be.
But this post is about how our roads are too big. How do we solve that? On collector roads, the ones we have now, can mostly have 1.75 metre (round up to 1.8 metres generally) to 2+ metre wide cycle lanes or 2-2.5 metre wide one way cycle tracks, sometimes 3.2-4 metre wide cycle tracks, we can widen the sidewalks, narrow the car lanes, the parking lanes, widen boulevards and planting boundaries, and make it more pleasant for all.
There are a couple other things needed to make them work right. They need bus stop inlets when you can, especially on the arterial roads that will be downgraded to 2 lane roads. Most arteries have more than enough width for this. Some routes even would benefit from completely separated lanes, but that is not always going to be the case. They still need separate bicycle paths away from cars. There is still considerable volume here, and there is still more than 30 km/h speeds. Speaking of speeds, the ideal speed limit for a distributor road in an urban area is 50 km/h. Pedestrian crossings will be made much easier, and bicycle crossings. The crossings can be set back at sideroad intersections, the speeds at the main road will be lower, and there will be median refuges, more zebra striped crossings, fewer traffic signals and less guessing what cars will do, and less guessing which lane a car is in, as there will often be only 1. The lanes will need to be narrower, as many roads are still too wide even with a single lane. There will sometimes be a median for most of the length, but not always.
Here is a nearby collector to my home: https://firstname.lastname@example.org,-113.515195,84m/data=!3m1!1e3?hl=en. It is a little skewed because it's right near a school, but we can account for that. Especially the desire to cross as few roads as possible, provide good links to the park and route the path directly to the bicycle parking, and bypass all of the hullabaloo near the school parking lot. And the tidal waves of bicycles. The parking near the school is not needed, there are no real destinations and there will be a parking lot at the school. There are no homes within hundreds of metres directly affronting the road, and it creates a risk to children, cyclists and creates a sightline hazard. The width as I measured at the bus stop is 21 metres wide. Lets create a cross section to fairly account for it.
Here it is: