According to the study a dehydrated driver is just as dangerous as a drunk driver. If you are mixing the dehydration with the excessive intake of alcohol, this could be double the trouble.
As part of the study, a range of tests were carried out over two days on male drivers using a laboratory-based driving simulator. During the normal hydration test there were 47 driving incidents, but when the men were dehydrated that number more than doubled to 101. These errors included lane drifting, late braking and touching or crossing the rumble strip or lane line. In fact, the number of errors made by dehydrated drivers was similar to that expected of someone driving under the influence of drugs or with a blood alcohol content of 0.08%, the current UK legal driving limit.
You can read more at http://www.thedrinksbusiness.com/2015/04/dehydrated-drivers-as-bad-as-drunk-drivers/
The truth of the matter is that we are mostly water in out physical make up, and we cannot afford to loos much of the volume of water before some very negative effects start to occur. Whether you are drinking or not, it is really important to keep yourself hydrated, because you could be as dangerous to people around you as someone that has been drinking excessively.