Cost. Some airports already have "stop bar" lights (green to enter runway, red to stop), or "follow-me" lights (follow the trail of green dots to your destinat

Cost. Some airports already have "stop bar" lights (green to enter runway, red to stop), or "follow-me" lights (follow the trail of green dots to your destination). But these aren't exactly cheap (nothing in aviation is), and installing them requires shutting down runways. They'll probably become more common at larger airports over the next decade or so.