The vehicle is equipment with a Stop/start when brake is applied and vehicle is at a stop, the engine turns off. Randomly the engine does not restart when brake is released and accelerator is pressed, vehicle is required to be put in park and restarted . This has happened on the highway while in traffic, the engine did not restart and required to be put in park and restarted while traffic flowed around me. The other occasion was came to a completed start at a traffic light, again the released the break and pressed the accelerator and the vehicle did not restart . Again had to put in park and restarted while traffic with cars honking . This failure has put us in danger. Vehicle brought to Buick dealer twice , same answer could not recreate, no codes displayed .