[bombcar]

Which is exactly what failed here, so saying "it shouldn't fail by not failing" doesn't help terribly much.

Having grade-separate crossings for vehicles might, but that introduces new issues (plane skidding off runway could hit the incline and break up).

[selectodude]

O’Hare has those but it’s not helpful for emergencies that happen on the runway itself.

[hypeatei]

Well, sure, but in that case it's expected that the runway is closed.

[selectodude]

The fire truck was responding to an emergency which is why it needed to cross an active runway.