Why is it 40’ kingpin length in Ca?

That CA Bridge distance is the perfect distance. As long as your load total isn't over and it's evenly distributed. You will weigh good.

 

TN changed their vehicle length law to allow 57 foot trailers for auto parts transport when Volkswagon built a factory near Chattanooga. Yeah, 57 ft trailers never seem to have caught on, thankfully.

 

A short and easy answer.... And the reason the cities have no interest in allowing the law to change.

To limit off track, which then gives the suburban planners/traffic engineers a turning path to work with when laying out curbs on road ways.

Especially important when you are updating corner curbs to wheel chair accessible ramps with a sweeping radius from mostly square.

Added benefit the same planners can then nix the idea of longer straight busses which would not work well in an established urban setting.

 

Like this fancy setup?