Authorities for years pulled people over for impaired driving without a breathalyzer. Touch your nose, look at the light, walk the line, say the alphabet backwards. 

This idea that we need a perfect machine and extensive regulations in place to deal with every possibly eventuality is rubbish. There's no evidence marijuana us impairs driving ability, and no reason whatsoever to think that existing impaired driving laws do not capture such impairment if it exists. 

Do the majority want laws just because they love regulations and red tape, or do you want laws that are sensible and effect some useful public purpose?