Siri may seem like the safest way to interact with information while driving, but according to a new report from AAA, Apple’s virtual assistant is a menace when it comes to the cognitive load it places on drivers.

The AAA Institute for Traffic Safety conducted a study on distracted driving that required drivers to operate different sorts of connected car systems while at rest and while driving, and then measured how hard their brains had to work in order to perform those tasks. Out of everything they measured, Siri required the most mental work to operate, and use of the service led to two crashes in simulated driving conditions.

In their report, Institute researchers laid the blame for much of the cognitive load squarely on Siri’s inconsistency in voice recognition. The report found that Apple’s assistant would perform different tasks based on the same voice command, even though study participants weren’t touching the device, and were speaking directly to Siri using a lapel microphone.

Of course, it’s possible that Apple’s virtual assistant may have been hurt by being able to do too much. Study participants using Siri were asked to perform a number of complex tasks including updating social networks and checking their calendars that weren’t available from other in-car systems that the study tested. But overall, AAA’s tests found that even speaking into a perfect voice recognition system required a moderately high amount of cognitive effort.

It’s also unclear how other assistants like Windows Phone’s Cortana and Google Now compare with Siri when it comes to distracting drivers, since they weren’t tested as a part of the study. However, it seems likely that they would rank in a similar place on AAA’s scale.

The news comes as Apple and other smartphone manufacturers are trying to push their virtual assistants as good tools for people to use while driving. Siri sits at the core of CarPlay, Apple’s connected car system, and the company advertises it as an “eyes-free” option for people who want to interact with their smartphone on the road.

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