In Seattle (where I lived for three years, until just recently,) there was this dedicated bus lane. Cars for the most part stayed off of it, but the bus still had to stop at traffic lights.
Anything on rail is just way more suited to nonstop use! People in cars can (or should) more easily understand that trains/streetcars can't just start and stop just for them.
Dedicated lanes and roads help, but light rail is significantly faster than buses & more reliable. It is a huge reason why we're investing in building Light Rail out to the outer reaches of Seattle.
With Light Rail, you are essentially buying people part of their lives back in the form of a shorter commute. That is quite important, as time spent driving in traffic is pretty well wasted.
Anything on rail is just way more suited to nonstop use! People in cars can (or should) more easily understand that trains/streetcars can't just start and stop just for them.