This is a description of a heavily car-oriented urban plan.
Ideally no urban street should have 40+ mph driving; it’s grossly unsafe for pedestrians/cyclists. Save those speeds for the highway. In my opinion cities should not have 6 lane streets. Such wide bands of pavement break up the neighborhood and are very unfriendly to everything but cars. Unfortunately, once you have built your city on such a plan with a 100 foot gap between buildings, figuring out what better to do with the space gets tricky.
A six lane, 40+ mph urban arterial street is a (significant) highway, though it may not be a controlled-access freeway (in practice, they have more controlled access than less significant urban streets, though often less than freeways, which also have higher speed limits; it's a continuum.)
I grew up in Southern CA, and the way cities there are crisscrossed by super-wide 6-lane roads (and every destination is surrounded by parking lots) is very disruptive to anything but a car-dominated lifestyle/culture. In many cities people almost exclusively travel by car, even though the weather is pleasant throughout the year. As a consequence lives are heavily scheduled and people spend little time talking to their neighbors.
I also spent significant time growing up in a colonial city in southern Mexico, designed around narrow streets with little available parking. The cultural difference was stark: in the Mexican city people walked everywhere and it was impossible to go anywhere without running into multiple friends in the street.
We generally don't. Arterial roads—what was being discussed—are typically between neighborhoods forming a widely-spaced network, from which a dense network of smaller roads branch off which are the roads “through the middle of city neighborhoods”.
That's what the word “arterial” means.
If side streets are available, they're great. But many places simply have none. And especially on bikes, most people prefer to get where they're going via the most expeditious route possible.
I enjoy the calm of side streets, but any route that is low in traffic is frequently (maybe even axiomatically) much longer or full of stop signs, both of which can be rather burdensome to a cyclist. E.g. I can bike to work 5 miles down the main road, or I can avoid cars almost entirely but it's 9 miles.
In my town, a six-lane arterial [...] turned it into a two lane road with center turn lane, bicycle lanes, and parking
A 40mph+ thoroughfare with parking added on both sides? That's so crazy I feel like we're missing something here. Does this road go right through the heart of downtown? High speeds are not really compatible with a downtown replete with bars, shopping, nightlife, & pedestrians.
It's about not getting lost when you travel someplace where you don't go often, at least for me.
I've never really understood the appeal of major roads for bicyclsts. I much prefer sidestreets when bicycling and don't mind cutting over a block to have to deal with less traffic. I think this demand along with a general blowback against cars due to American culture treating non-vehicle traffic as second class leads to bad outcomes.
In my town, a six-lane arterial connects neighborhoods with a major N-S arterial, downtown, and past downtown is a US highway. The town has taken that six lane road and turned it into a two lane road with center turn lane, bicycle lanes, and parking. What should be a 40+ mph speed limit is now a 25 mph speed limit.
This has bad outcomes for drivers and bicyclists.
Effectively, from the perspective of a driver, this road is now equivalent to any of a dozen parallel roads: single lane in each direction and 25 mph. This disincentivizes drivers from cutting over to the arterial, effectively spreading out the traffic onto parallel roads (many of which lack traffic lights when they hit the major N-S artery). However, it remains the busiest road for vehicle traffic, and because it is so wide, speed significantly faster than 25 mph feel safe, so speed variance can be higher, with traffic typically traveling between 20-40 mph. It's particularly sketchy for traffic turning on from side streets. Drivers expect traffic to be going the posted 25 mph, but are sometimes surprised by the person going 40 mph. So, drivers generally get where they are going more slowly, but at increased danger due to increase in speed variance.
But it also increases danger for bicyclists in two ways. First, it encourages the bicyclists to use the still relatively busy arterial. It's not clear why anyone would want to incentivize this. For a bicyclist, any of the parallel streets were already attractive options and would get the bicyclist where he was going. Second, because of the changed incentives, those parallel roads are now busier than before (making the bicycle lane on the busier road more attractive). Whatever a bicyclist decides, there is an increase in vehicle/bicycle interactions.
A much better solution would have been to keep the arterial an arterial and make dedicated bicycle lanes on some of the parallel roads. This would encourage a separation of vehicle and bycicle traffic that wouldn't really inconvenience either one of them.