What he said....Ah, 18mph must be autosteer? Or is that a total bs number?
You need to be going 18MPH to engage the system on a clear road. When there's a car in front of you, there's no speed requirement. I've often engaged TACC+Autosteer while waiting at a stop light.
Also you can't set TACC to a speed lower than 18 MPH.
Engaging TACC at a stoplight used to be my way of letting the car keep itself stopped before they introduced the hill hold feature.