For cars: 1. Make sure your tyres have enough tread on them or you will run the risk of slipping or aquaplaning (when a layer of water builds between the wheels of the vehicle and the road surface, leading to loss of traction).
2. Worn-out pads and liners in a bike brake can reduce the efficiency or ‘bite’ of the brakes. In the case of drum brakes, get them cleaned from the inside, and in the case of disc brakes, check the brake fluid. Also, keep the discs and callipers clean and mud-free.
3. Make sure you inspect headlights and all electrical mechanisms like power buttons and speed indicators before you ride the bike on a rainy day.