Distance between noisy power lines and sensitive signal tracks depend on the strength and spectrum of the emitted noise (not the DC current) as well as the impedance and noise margin (difference between 0 and 1 voltages) of the target signals.
There is no exact solution, it is an empirical problem.
I can give you some general rules:
1 - keep noisy and sensitive traces as apart as possible and avoid running them parallel for long distances (ideally crossing directions)
2 - Have at least one ground plane between power, noisy signals and sensitive ones. If this is not possible, have a ground guard as wide as possible between them
3 - improve your design to minimize HF currents and minimize emissions
4 - strengthen your sensitive lines with lower impedance drivers and higher noise margin.
An additional design consideration is the thermal one. 18 amps needs already some mm wide traces. Google for "trace width calculator" or similar and use one of them. When changing layers, use more then 1 via. It is different for inner and outer layers and it depends on the ambient temperature (the hotter the wider the trace). To be in the safe side, be generous with the result and increase it by a 10 - 20%. Your PCB life will be longer.