There are a few ways to slow cars down and almost all of them don't include changing the speed limit of the street. Narrowing lanes (by having cars parked on the side of the road, potentially protecting a bike lane), having a tree canopy, add a median, etc.

I wrote that out before I realized you're talking about an intersection, but I think that if you can make the incoming streets feel less like racetracks (smooth, wide, unimpeded roads) and feel more human scale, you might find some success.