What does 50 on a blue background mean?

This is a road sign that says "the minimum speed limit is 50 kilometers per hour". The following is an introduction to the minimum speed limit: 1, meaning and examples: Road traffic signs are facilities that use graphic symbols, colors and words to convey specific information to manage traffic. It plays a role in ensuring smooth roads and traffic safety, so drivers and traffic participants must abide by it; Traffic safety propaganda slogans can improve drivers' and traffic participants' awareness of consciously observing traffic laws and regulations, which is of great significance to building a safe and smooth traffic environment. 2. Laws and regulations: According to Article 78 of the Regulations for the Implementation of the Road Traffic Safety Law (hereinafter referred to as the Regulations), the expressway shall indicate that the maximum speed of the lane shall not be higher than120km and the minimum speed shall not be lower than 60km. The minimum speed of the left lane with two lanes in the same direction is 100 km.