Metros are the only driverless systems in mass transit so far. There are some ongoing projects for driverless freight but I consider that it is not relevant to this particular discussion.

I don't want to sound obnoxious but driverless metros routinely operate in the open air :). Tunnels are not a mandatory condition for such systems. They manage strong winds, strong rains with reduced adhesion conditions, etc. They manage the possibility for people or non-protected trains to intrude on the track. A lot of brain was put into automating decisions related to fire scenarios or emergency evacuations. Metro lines like Paris line 1 are extremely busy. There are plenty of ways to kill people, yet they don't. Just the "stuck hand-bag" scenario is a nightmare to manage. Yet.

A driverless system is truly a massive piece of software. You guys should come in the tunnels and have fun with us!