Plain glass does offer some protection against UV, 75% or so is filtered out, it depends on the thickness of the glass. So you can get sunburnt even though you are protected by glass, it just takes a longer exposure time, which is why this can happen on long road trips. Also you would have to take into account how well your skin copes with UV, which varies from person to person.
Regarding glass, UV and sunburns
Plain glass does offer some protection against UV, 75% or so is filtered out, it depends on the thickness of the glass. So you can get sunburnt even though you are protected by glass, it just takes a longer exposure time, which is why this can happen on long road trips. Also you would have to take into account how well your skin copes with UV, which varies from person to person.