It should be legal, it would make millions in taxes and save millions by not policing it.
I've only looked at 4 tests of drug driving,
Test 1 they ended the test when the driver was doing better than his sober try.
Test 2 the driver was better than sober and the TV presenter (if I remember right was Quintin Willson) guessed it was because he was concentrating harder but after a large smoke he was still more than good enough to pass a driving test.
Test 3 the TV presenter who had never smoked before got stoned and was driving badly but she looked like she was probably a bad nervous driver before smoking.
Test 4 they tested 4 or 5 drivers and the regular smokers driving improved and the non smokers got worse.
It seems wrong that with no real proof that smoking can cause accidents that you can have a smoke then a week later get tested and loose you're driving licence, job or both.
They need a test like in America where you touch your nose walk along a line etc to judge how affected you may be especially as the current test is a waste of money as I passed 4 hours after a smoke yet a mate failed 2 weeks after smoking, if you smoke you need to know how to pass the test.
I have driven a rally car stoned and done a motorcycle track day where I was complemented on how smooth and fast I was and my only accident on the road was after 6 months with no drugs or alcohol I went round a blind bend and there was a German driving on the wrong side of the road.
Smoking and driving isn't the big sin most people believe, too many people listen to the lying government and media and blindly believe those lies.