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.
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.