The rising popularity of mixed drinks like the "Jager Bomb" (Jagermeister and Red Bull) or the self-explanatory "Vodka and Red Bull" has worrying implications for road safety, research shows.

A field study of more than 800 young drinkers in the US has found those who consumed caffeine-alcohol mixed drinks were often in a drunker state when they left the bar.

Compared to those who drank only alcohol-based drinks, those who reported having caffeine-alcohol mixed drinks were also much more likely to say they intended to drive home.

The problem stems from consuming a stimulant alongside an intoxicant, which clinical studies have shown can reduce the perception of being drunk but not the impairment.

The University of Florida study put researchers on the streets outside bars and clubs, from 10pm to 3am, to quiz young revellers on what they had been drinking and assess their sobriety.

"Patrons who had consumed alcohol mixed with energy drinks were at a three-fold increased risk of leaving a bar highly intoxicated," the study found.

"... As well as a four-fold increased risk of intending to drive on leaving the bar district, compared to other drinking patrons who did not consume alcoholic beverages mixed with energy drinks."

Red Bull's introduction to the US market in 1997 has lead to an explosion in the global energy drink sector.

Consuming these drinks with alcohol was now emerging as a cause of "elevated involvement in night-time risk-taking behaviour", according to the study which is published in the journal of Addictive Behaviors.

The rising popularity of caffeine-alcohol mixed drinks was not confined to the US, an Australian expert said.

Dr Lucy Burns, from the University of NSW's National Drug and Alcohol Research Centre (NDARC), said a 2009 study of almost 700 Australian ecstasy users found about 70 per cent had consumed caffeine-alcohol mixed drinks.

"It is common to believe that energy drinks will reduce the fatigue, cognitive and motor impairments of alcohol and other drugs," Dr Burns said.

"So there is a real danger that consumers may be more likely to engage in risky behaviours, such as operating a car or a motorcycle."

Consumers needed to be more aware of the effect of caffeine on their body and the risk posed by consuming it alongside alcohol, she said.