Crossings

Offhand, there are bare handulf ways for a transit line to cross a roadway.

• Grade-separated
• Overpass/viaduct/elevated structure
• Underpass/tunnel/escavation
• Level-crossing
• Time separated
• Railroad Gates
• Traffic Signal

• A metro is a system where both the running way and crossings are grade-separated. 
• A pre-metro is a system (or line) where only the crossings are grade separated. 
• 'Rapid Rail' is the pupae form between metro and pre-metro, where the most urban sections have all their right of way grade separated. The M/ART systems did this.
• 
  

Both tend to be heavy rail systems. Or rather, heavy rail systems tend to be either metros. While its well known that 'you can't steer a train', it's less well known 'You can't stop a train' (at least in any reasonable period of time. Ie, if you park a vehicle on the tracks, by the time the driver sees it, it is too late to stop. "With so much steel and steam, I won't slow it down at all". This has been the cause of a number of memorable train crashes, and much conductor trauma. 

And that is the problem with level crossings. Grade-separated crossings remove the 'conflict' between things, with certainty, but are costly to construct. (They have to be able to support a bridge filled with full-loaded semi-trailer trucks, while never exceeding a 5% grade, and being as wide as the roads leading to them). 

Level crossings are much much less safe. Rather than being grade-separated, they are time-separated. When two moving vehicles could be in the same place, this is known as a 'conflict'. When two vehicles attempt to be in the same place, this is known as a 'crash'. Playing 'chicken' with a train ends badly--the train is never going to swerve. Yet drivers managed to run red-lights, and drive around lowered freeway gates with depressing frequency. (It is rare for the driver to survive). 

Transit Ridership History in the US

From here

Puts things in context, eh?

Heavy rail remained remarkably constant across all time periods, though. In many cases, for many destinations, heavy rail transit is simply better than the personal automobile.

One issue I have with this graph is that it lumps 'light rail' in with trolley. They aren't quite the same animal. Trolleys are uniformly street-running, in mixed traffic. What APTA calls 'light rail' (post 1980) typically uses that type of Right of Way for only a portion of it's running-way. Most light rail systems use a mix of separated, exclusive, and mixed. A portion of freight railway, or the median of a freeway provides separated part, and exclusive and mixed right of way on public roads provides the rest.

 As Meyer/Kain/Wohl pointed out in the Urban Transportation problem (50 years ago), using a freight line with railroad gates is functionally equivalent to a heavy rail. The big different between the two is that heavy rail systems use tunnels and light rail doesn't.