On a road where the **maximum speed limit for a passenger car is 90 km/h**, the **minimum speed limit** is half of that speed, which is **45 km/h**. This means that any speed below 45 km/h is considered **abnormally low** for a passenger car on that road.

- **Minimum speed**: It is the lowest speed at which a vehicle must travel on a specific road. If a vehicle travels below this speed, it is considered to be traveling at an abnormally low speed.

- On roads that are neither highways nor expressways, like the one mentioned, the minimum speed limit for a passenger car is half of the maximum speed limit. Therefore, on a road with a maximum speed of 90 km/h, the minimum speed is 45 km/h.

- **Exceptions**: There are situations where driving below the minimum speed is allowed, such as in cases of heavy traffic, breakdowns, poor road conditions, or if the vehicle is special or accompanying other vehicles, like in cycling competitions.

Therefore, on a road with a maximum speed of 90 km/h, any speed below 45 km/h is considered abnormally low for a passenger car.