I sent my boyfriend a WeChat message, 15 minutes later, he replied that I was driving, probably waiting for the traffic lights. Does this mean that he has me in his heart?

Yes, he has you in his heart. Whether a man loves you or not depends on the speed of SMS reply. But you have to tell him that you must not turn on your mobile phone while driving, let alone return to WeChat or make phone calls. Even when waiting for the red light, I will be distracted. In the long run, we can't judge whether we love you or not by this, but we need two people to get along slowly and run in with each other in order to have a happy life.