Driving at **120 km/h** on a motorway in **heavy rain** is an **inappropriate** speed. Although this speed is within the permitted limits under normal conditions, adverse weather conditions, such as heavy rain, require a reduction in speed to ensure safety.

- **Reduced visibility**: Heavy rain makes it difficult to see the road, which can increase the risk of accidents.

- **Reduced tire grip**: In the rain, tires have less grip on the road, meaning the vehicle needs more space to brake safely.

- **Safety recommendations**: It is advised to reduce speed, keep tires in good condition, and increase the distance from the vehicle in front to avoid accidents.

For these reasons, driving at 120 km/h in heavy rain is not appropriate, as it does not adjust to the weather conditions and increases the risk of accidents.