Personally I don't think slowing drivers down is the way to go. I've been a driver and I hate it when the traffic moves slowly and when I have to weave my way through traffic slowing devices etc. etc. It makes me irritable. And the last thing cyclists need are irritable drivers.
IF the government is going to look at slowing traffic down on one street, they need to make sure there is another option for drivers to move briskly.
In my ideal city plan there would be a dual carriage way (or wider) ring road all around the edge of the city with wide shoulders so that both cars and cyclists could cruise along together ... and the cars could do 100 km/h out there. There would also be at least one main North/South road, and one main East/West road ... the larger the city is the more of these there would be. These would also be at least dual carriage ways or wider with wide shoulders so that both cars and bicycles could be on the road together without getting in each other's way. The speed limit on these roads might be 80 km/h.
I like the grid system so the Northwest subdivision would be crossed with a main North/South road and a main East/West road. These would still be dual carriage way, and would have shoulders for cyclists, but the speed limit might be lower ... 60 or 70 km/h. The same would be true for each of the other subdivisions.
And then there would be the smaller residential or industrial roads within the subdivisions. These could have some traffic slowing devices if necessary, but should still mainly be wide enough for both cars and bicycles to travel alongside together. I saw a particular traffic slowing device I liked in ... England, I'm thinking. The motorized vehicle opening was just wide enough for one vehicle to pass through at a time so if there were another one going the other way it had to give way. But there was another opening about the width of a sidewalk for cyclists. So a car and a bicycle could both co-exist quite happily together.
This sort of set up would allow motorists to get from point A to point B quite quickly with fast roads to get them from one subdivision to another. They may have to slow down at the beginning of the journey and at the end, but the middle of the trip could be quite fast. And it would allow bicycles to be out there with the motorized vehicle, but out of their way.
(I could draw a diagram of the plan I'm thinking of if what I've said is not clear.
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