After one joint, marijuana smokers have difficulty recognizing their surroundings. Time and driving speed judgments are impaired. Peripheral or side vision and central vision are both affected by marijuana use, making it more difficult to see another car trying to pass. A driver may not remember to start the car when the traffic signal turns green, or fail to acknowledge signs. The intoxicated driver will have impaired night vision which means the driver will take more time to recover from glare.
Lights are more difficult to place in space.
Physical coordination skills are also reduced so that the driver has difficulty in turning, backing and maneuvering. Often marijuana users believe they are in control of a car and feel quite sober. Some believe they drive better when stoned. This is a dangerous illusion. What's more, the impairment level of the driver actually goes up as the user feels he is coming down from the high.