Single decker buses roofs

[tali1]

Single decker buses have a ventilation box ? on roof and several other boxes on their roofs -what are they? - and why do Double deckers not have them?

[SlackAlice]

As far as I recall, single deckers may have ventilation/ air conditioning units on top, plus additional add-on boxes for radio transmitter/receiver and gps locator/identifier transmitters. They are put on top to minimise intrusion to passenger space.
On double deckers they cannot put them on top because of there need to get under road/rail bridges. To put them on top means the passengers headroom would not be sufficient on the top deck. They have a much smaller dome about 3" high to cover radio transmitter/receiver.