The incident took place after a Cars and Coffee event around 11 a.m. Saturday morning. Investigators say he lost control and ran into the nearby crowd. The people hurt had serious but not critical injuries, said Boise Police Lt. Charles LeBar.
Cars and Coffee is a monthly event, between April and September, where auto enthusiasts gather together to "share their passions", according to the event website.
The driver, who is cooperating with police, has not been identified.
"We do not have any information that this was an intentional act", Lebar said.
Eastbound Overland Road remains closed from Entertainment Way to Cole Road. The driver then lost control "while going too fast", went up over the sidewalk and onto the grass where onlookers were sitting.
Police have confirmed at least 11 people are injured after a Porsche Boxster Spyder jumped the curb and crashed into a crowd in Boise, Idaho. "And after that it was sheer screaming".
Authorities said an investigation is underway and the evidence will be sent to the prosecutor for "possible charges".
On social media, there are unconfirmed reports from witnesses suggesting that children were among those hit. "Took six ambulances, five (taken from the scene) private cars", - reported on the website of the police in Twitter.
This was the last Cars and Coffee show of the season, according to the event's Facebook page.