Consider this the next time you’re tempted to call the office as you fight traffic: Listening to a cell phone call can impair brain function and lead to dangerously erratic performance behind the wheel.

Common remedies intended to make on-the-move calling more safe, such as voice-activated dialing and hands-free headsets, don’t get to the source of the problem. Just listening to a phone conversation reduces the amount of brain activity normally devoted to driving by 37 percent, according to a study by Carnegie Mellon University’s Center for Cognitive Brain Imaging (CCBI).

The 29 volunteers who participated in the study were placed inside a magnetic resonance imaging brain scanner equipped with a driving simulator. They were then asked to negotiate a winding road at a steady, “challenging” speed, either without distractions or while answering true-or-false questions. The simulator results showed that while the drivers were listening to the questions, they were more likely to swerve across lane lines or crash into simulated berms and guardrails.

“Comprehension of a spoken language that you understand is the highest level of cognitive processing a person can do, and you can’t stop it,” says Marcel Just, CCBI’s director and professor of psychology at Carnegie Mellon.

Conversation from a passenger in the vehicle could have a similar effect, but passengers are typically aware of competing demands on the driver’s attention and suppress conversation when the situation requires the driver to concentrate.

“Everybody multitasks sometimes, but you have to choose when you do it, especially when speech is involved,” says Just. “Driving through an intersection is not a good time to multitask.”