I went looking for this a year or so back and I couldn’t find any big study that looked at the lifetime emissions of motorcycles compared to cars, but the road emissions by themselves are generally about as bad because while there are fewer emissions from motorcycles, the emissions that exist are worse because the smaller engine doesn’t burn the fuel as completely. The thing is a huge chunk of a car’s lifetime emissions come from manufacturing, which I suspect makes motorcycles better, but again I never found that comprehensive study.