def calculate_total_number_of_travel_requests_in_timetables(timetables):
    """
    Calculate the total number of travel_requests in timetables.

    :param timetables: [timetable_document]
    :return: total_number_of_travel_requests: int
    """
    total_number_of_travel_requests = 0

    for timetable in timetables:
        travel_requests = timetable.get('travel_requests')
        number_of_travel_requests_of_timetable = len(travel_requests)
        total_number_of_travel_requests += number_of_travel_requests_of_timetable

    return total_number_of_travel_requests