Often, there are many solutions to a problem, with different externalities and implementation requirements. Speed limit enforcement for sure has some benefits, but also some downsides. Any absolute has edge cases that don't work, but I don't in fact think creating the large networks of people empowered to surveil citizens to enforce speed limits is the only viable means of reducing traffic deaths, a goal which is often better served by creating more viable transportation alternatives to cars. I don't think carceral approaches to problems are always wrong, but they are often an attempt to individuate a problem that's better addressed on the level of infrastructure