the simplest solution
No law - at least as pertains to drivers -
New law :
All cellphones and other related texting/phone call capable devices - MUST NOT FUNCTION for incoming or outgoing service - with the exception of the local emergency service(s), if the device is moving more than (name a reasonable slow speed - I use 10 mph) .
This allows walkers, joggers, and grid locked drivers to make and manage phone calls, texting sessions etc. -- However once the phone starts moving faster (ie the user is driving) - existing calls are disconnected, and no new call can be stared or received.
All of these devices already have either GPS or equivalent capabilities so determining the current travel speed is not an issue.
I biggest objection I hear from people I propose this Idea to is : what about passengers?
my answer is: shag them -
(in the US at least) as a passenger you not allowed to drink alcohol (in the vehicle - open bottle laws) even if the driver is not drinking. On public transport - well that just makes an already bad experience just a little bit better without having to listen 95 different idiots on the phone.
Flames - cause I expect to get a few.