Governments would build infrastructure to support it. In the same way that we allow people to drive 2 ton vehicles with minimal training and minimal local communication on marked roads, broom drivers would be given similar treatment.
As long as everyone follows the rules, then there need be no additional communication.
For instance, you might have 25 foot or 8 meter height divisions. At 0-25 feet you are to travel no faster than 15mph (24kmh) to facilitate landing and takeoff from any location. At 25-50 feet you can only travel within 22.5 degrees of north, at 50-75 north west, 75-100 west, and so forth. To launch you always start north, and then accelerate in a circle as you ascend to the direction you want to travel, and these initial lanes are fairly low speed. Higher lanes are similarly broken up, but with much higher speeds and much more space between differing directions to avoid accidents with people traveling in opposite directions. Minimum speeds will be necessary in all cases, because sight lines won't be very good and a fast moving broom could overtake a slower moving broom too quickly to avoid an accident.
In lighting isn't available, hand signals or simply body positions could be enough. Leaning back to slow, forward to accelerate, tipping the head one way or the other and up and down to indicate a turn and ascending or descending.
Markings on the ground could provide further special notices, lanes, etc. For instance there may be a special height that tracks into the city center in the morning that changes into a track going out of the city center. If you're at that height you need to be aware of the time, and you can only travel directly toward the city, regardless of your current location, during certain times, then cannot use it during a switchover time, then must only use it to travel directly away from the city in the evening.