Even the lightest of rain showers can make the road surface slick and slippery as the water mixes with oil and rubber dust.
On water covered surfaces your car can hydroplance. That's when your tires ride on top of the water like a water ski. The faster you go the more likely you are to hydroplane, but this dangerous effect can occur at low speeds too.
Travel more slowly when the road is wet.
Be prepared for loss of steering control especially around corners and curves. Brake before entering the curves.
Make sure your tires have ample tread depth and maintain the correct air pressure.
Be sure your windshield wipers are working efficiently and you have plenty of washer fluid. Carrying extra can be very beneficial during the winter.
Make sure your lights are on and clean.
I know that splash and spray from trucks can be a worry. Recent improvements in vehicle design are helping and the industry is working with truck manufacturers to reduce the effect even more. In the meantime keep extra distance, and your windshield wipers on when other traffic is around.