Why do many cars have lights on during the day?

This lamp should be the daytime traffic light. 

A daylight is a lamp that makes a vehicle more easily recognized during the day. This lamp is not a lamp, not to make the driver see the road clearly, but to let others know that a car is coming. It belongs to the category of a signal lamp. The daylight should meet the basic brightness requirements, but it should not be too bright, so as not to interfere with others. The daylight is different from the ordinary low-light lamp. It is specially designed for daytime driving illumination. The daylight lamp uses LED technology. The energy-saving effect is further improved, and the energy consumption is only 10% of the ordinary low-light lamp. As soon as the car engine starts, the daytime traffic lights automatically turn on and continuously increase brightness to attract the attention of other motor vehicles, non-motor vehicles and pedestrians on the road. When night comes, the driver manually turns on the near-light, and the daylight automatically turns off. The function of daytime traffic lights is to provide vehicle identification. Driving in foreign countries with daytime traffic lights can reduce 12.4% of vehicle accidents and 26.4% of vehicle accident deaths. In short, the purpose of daytime traffic lights is for traffic safety. Therefore, in recent years, many countries have formulated relevant indicators of daytime driving lights to ensure that the production and installation of daytime driving lights can really play a role in ensuring safety. The most representative one is ECER79 in ECE automobile technical regulations of ECE. Our country also began to implement the national standard "Light Distribution Performance of Automobile Day Driving Lamp" issued on March 6, 2009 on January 1, 2010. 

The most important point of LED daytime traffic light is its light distribution performance. The daytime traffic light should meet the basic brightness requirements, but it should not be too bright, so as not to interfere with others. In terms of technical parameters, the luminous intensity on the reference axis should not be less than 400 cd. In other directions, the luminous intensity should not be less than 400 CD and the percentage product of the corresponding points in the distribution chart. In any direction, the luminous intensity emitted by the lamps should not be greater than 800 cd.

Let me give you two pictures to see the difference easily.