Idaho State Police at the scene say the crash was caused when the first car struck a jersey barrier for unknown reasons.
A car driving behind the first car slowed down after seeing the crash, causing a third car to hit it from behind, according to troopers.
Ada County Dispatch says at least one person has been transported to the hospital with non-life threatening injuries.
All lanes of the ramp were closed while crews responded. They reopened around 8 p.m.