Medical research has long suggested that most of us go through life using only a small percentage of our potential brainpower.

Just look at Rush Limbaugh, who readily admits that he makes a living with half his brain tied behind his back.

And according to studies unveiled recently, talking on a cell phone while driving reduces that limited brain activity even further, which can compromise our ability to react to unpredictable events on the road.

David Strayer, a University of Utah psychologist, tested the effects of cell-phone use on 500 adults operating driving simulators. As he told a meeting of the American Psychological Association last week, the studies showed that talking on a phone cuts a driver’s brain activity by half in the critical area of the brain necessary for monitoring traffic conditions.

By tracking eye movements, Strayer found that drivers were far less likely to recall seeing something in front of them if they were on a phone at the time. When our brains are fully engaged in a phone conversation, our eyes can look directly at an object – a road sign, the brake lights of a car – and not really see it at all.

“They’re as blind to dumpsters along the road as to a child running across the street,” Strayer told the USA Today newspaper .

Because the brain has only a limited capacity for attention, he said, diverting a portion of it to unnecessary phone conversations reduces our ability to concentrate sufficiently on the most important task at hand – getting from one place to the other without endangering ourselves or other motorists around us.

The research found that having a conversation with a passenger also can reduce a driver’s attention, but not nearly as much as talking on a phone. In fact, he said, passengers tend to help drivers by pointing out hazards ahead or simply by shutting up when a situation requires complete concentration.

A companion study by University of Kansas psychologist Paul Atchly suggested that while all phone conversations diminish a driver’s focus, the emotional ones – especially those that concern unhappy issues – are the most distracting of all.

Strayer, who has conducted similar lab tests on 18-to-25-year-olds, found that drivers slow down when their attention is being funneled into phone conversations. His young test subjects tended to drive sluggishly while on phones, allowing the cars in front of them to pull far ahead. Yet when the car in front braked in heavy traffic, several of the distracted young drivers plowed right into their rear ends.

“Twenty-somethings on a cell phone have the same reaction time as 70-year-olds, and older drivers are super-slow,” said Strayer.

And in case you’re wondering, hands-free phones, which are mandated in Connecticut, New Jersey, New York and more than two dozen local communities around the country, pose the same safety hazards as the handheld variety.

As a mounting body of evidence offered by Strayer and other researchers has made clear in recent years, the most dangerous aspect of behind-the-wheel phone conversations is not the distraction of holding the phone but the mental disconnection caused by focusing on the conversation itself.