Batman:

 

Does Canada allow driving and smoking weed?

 

Nope. It's already covered under 'impaired driving'

But Canada's using this as an opportunity to “reform the entire Criminal Code regime dealing with transportation offences, including impaired driving” with legislation to be passed before legalisation next year.
They're toughening the penalties for drink-driving, but for THC (the main psychoactive compound in cannabis), the proposed levels would be:

2 nanograms (ng) but less than 5 ng of THC: Having at least 2 ng but less than 5 ng of THC per millilitre (ml) of blood within two hours of driving would be a separate summary conviction criminal offence, punishable only by a fine.
This lower level offence is a precautionary approach that takes into account the best available scientific evidence related to cannabis. This offence would be punishable by a maximum fine of up to $1,000.

5 ng or more of THC: Having 5 ng or more of THC per ml of blood within two hours of driving would be a hybrid offence.
Hybrid offences are offences that can be prosecuted either by indictment, in more serious cases, or by summary conviction, in less serious cases.

Combined THC and Alcohol: Having a blood alcohol concentration of 50 milligrams (mg) of alcohol per 100 ml of blood, combined with a THC level greater than 2.5 ng per ml of blood within two hours of driving would also be a hybrid offence.

This is more complex than say, Colorado, where - driving under the influence of pot: Any driver found with more than five nanograms of THC can be charged with a DUI.