The regulations require an emergency exit from the cockpit, even if the cockpit door is blocked. Kickout panels in the door are acceptable, but a seperate exit is clearly better.
If the cockpit is equipped with sliding windows (as it is for most large transports) this provides an additional exit.
For smaller airplanes like the Canadair CRJ, it is hard to embody a window frame that allows some cockpit windows to be opened. Therefore these planes are equipped with escape hatches in the cockpit roof. Some even have both, with the sliding windows optional (as for the
747-400, don´t know for the older ones) and therefore the hatch as a standard.
Here you can see the escape hatch of the L1011, and the inertia reels (yellow) for up to 5 people to get from the hatch to the ground.
I don´t know whether the flight manual allows to use the hatch for a
lookout 