When designing a flying car intended for congested city routes with slow traffic, should turn signals indicate all possible directions. In most sci-fi, I've seen designs that have no turn-signals or the same left/right signals that a normal car has. Imagine the scenario of a car flying along in a lane at 300 feet, if it would like to reach a lane a city block over to left at a higher altitude - it would need to turn diagonally upwards to merge into the new lane. This problem could require 8 directions for the signal. Up, down, left, right and the four diagonals. One design for this system could be a circle surrounding a rear central thruster which would be broken up into 8 light pieces that flash when the pilot puts on the appropriate indicator. How would you design a turn-signal system, and what considerations might I be missing?
One thing you need to keep in mind is whether if such an indicator is even useful - ground driving requires checking 4 directions (ignoring blind spots) to see someone else and their turn signal. If they can be above or below, suddenly there are 12 directions minimum to check. Chances are blinker lights will not work out that great in your situation.
Now one can instead have a car transmit their intended direction to the drive computers (which might be as necessary as an engine on the thing) in their immediate vicinity, which can then plot and display and/or warn about a lane change on some sort of HUD.
As for how this signal would be transmitted in the first place, how about the 'wheel' itself? If a flying car is supposed to keep a lane, chances are some automatic process should be involved. It is one thing to keep aligned between two lines you can see in front of you, and another thing to keep aligned on a lane in the sky. This means there is at least a semi-auto driving mode involved where you need to input what you want to do and have it figure it out.
At a bare minimum this would be turning your 3D wheel in the direction you want to go and have the drive computer figure out the best approach to switch to and stay in a lane. While it is doing that, it can broadcast your intent to the nearby cars, drive computers on which would account for this info when their drivers want to switch lanes or turn, slow down if needed etc.
All of this is assuming you are fine with your cars being autonomous at least to some degree, in which case turning signals would more than likely be a given in the process any AI would use. If that is not the case, you might have bigger concerns with a sky-highway before thinking about turn signals.
Manual control of 3-D movement in a congested area is a recipe for disaster, at a rate of dozens a day!
If traffic is sparse enough to permit manual control, then signals won't matter or be of any help, any more than they are now on a Cessna or Piper light plane, or even a Bell Jet Ranger helicopter.
Even if cars can be drive/flown manually when in "open areas", if they're in an area congested enough to see multiple other cars at any given time (not just rush hour, hover-and-go levels of density) networked control would need to be mandatory -- to the point the car would override any attempt to enter a controlled area under manual control; even when in "manual" you have a fly-by-wire system, where the "pilot" or "driver" is simply telling the car where they want to go, and is not actually in direct control. Flying under direct control, in a mature system of this sort, would be a felony, seen as a lead-in to terrorism (well beyond recklessness).
The cars would fly themselves, and keep track of each other (or be directed by a central control authority). No visual signals (or even windows!) would be needed while the cars are in "controlled traffic" mode.
Turning signals will be transmitted to the Heads Up Displays of the cars that are around and will be displayed as arrows that move in a circle.
The signal controls will be either triggered by
- The driver's mind (by just thinking that they need to move to one direction).
- Using special glasses or contact lenses. The driver just needs to look at the direction they want to turn and the turning signal will start to transmit to the other drivers
Of course, after the signal is sent, the nearby cars computers using GPS or some other technology interpret the signal sent from the car that wants to turn to an arrow which is relative to their position.
Take a page from airplanes. Usually they are flying at such distances and speeds, and in such weather conditions that "turn" signs would be meaningless. Also their paths are controlled by ground towers.
Driving is already complex enough as is. Humans simply won't be able to drive a flying car as we used to see in the Hanna Barbera's Jetsons cartoon. Most likely cars will be self-driving. Humans will just input where they want to go. The AI's running the cars can then use wireless communication to decide when and how they will turn, and how to give each other the space they need.
This is not the stuff of fiction. The IEEE was working on a protocol for intervehicular communication in the last decade, built on top of 802.11 (the same bases for wifi). 802.11p would be called "wave" (Wireless Access in Vehicular Environments). 5G networks will surely resurrect interest in this technology in the next couple years. And Uber, Lyft etc. are investing in self-driving cars (they'll be more profitable once they don't have to share income with app drivers), so self-driving cars may become really ubiquitous before we have reliable flying cars.