I’ve suggested using up excess street width with diagonal parking. Below are two photos of 17th Street in midtown, one of the section between N and O, which has parallel parking, and a much-too-wide street width. The second is between O and P, which has diagonal parking on the west side. Since these are right next to where I live, I get a regular chance to observe the behavior of drivers on these two sections. On the parallel parking section, drivers are almost always moving above the speed limit, about 30 mph, particularly since most of them have come from an overly-wide section of 17th north of N Street. On the diagonal parking section, drivers are almost always moving at less that the speed limit, about 20 mph. A more subtle difference is that northbound drivers, from the diagonal parking section, seems more willing to yield to pedestrians at the unmarked crosswalks at the intersection of 17th and O streets, whereas southbound drivers, from the parallel parking section, seems to be less willing to yield. Narrowing streets with diagonal parking really does make a difference!
Diagonal parking should be of the reverse diagonal type, which is also called back-in parking. To use these spaces, the motorist passes the parking spot and then backs into it. Since they have just looked at the space, it is much less likely that they will run into or over something as they back in, whereas when pulling out of front-in parking, the motorist has very poor visibility and may well run into or over something. So for bicyclists, reverse diagonal parking is much safer. And it is somewhat safer for the motorist themselves. The photo shows reverse diagonal or back-in parking on 29th Street in midtown Sacramento.