Not so...I'm no optician but i do not think sunglasses restrict your sight in the same way,if at all,as darkened glass..
Having had some prescription sunglasses made at specsavers (other rival opticians are available
The highest of the three (called class 4?) is illegal to drive with.
"According to the AA: “Filter category 4 lenses only transmit between 3% and 8% of light and are not suitable for driving at any time.
If you have these type of sunglasses, then it is illegal to use them while driving...”

