going off highway

If you have your preferences set for shorter distance it will do that.

It can also do that if you have it set to avoid highways, but have a setting for 'faster time'. As you get to the roadways with alternatives (the exits) it can route you off, and then right back on.

The T means it has traffic on it right? Maybe there is a traffic jam it has you avoiding.

That's likely the reason, also check under your choices for avoidances and if you've checked off traffic, uncheck that.

A few months ago, I read here that someone suggested unchecking Traffic for Avoidances. The next time I drove down to Phoenix (my nearest traffic-ed city), I made a route to a motel near the airport and a warning popped up saying that an exit ramp I needed about 14 miles ahead was closed but nuvi kept me going towards to closed ramp. Only after re-enabling traffic did I get an alternate route to avoid the closed ramp.

So for me, now I keep traffic enabled. On the days when there's a slow-down ahead of me and nuvi tries directing me off, then right back on the Interstate, I just ignore the exit/entry suggestion.

If you had tapped the traffic icon that popped up on the screen when the alert sounded, you would get the options to get more detail about the alert, and you could then tap the Avoid button if you choose to avoid it.

I find this a more intuitive process than having it set to auto-avoid in the menu. Sometimes it's responding to alerts that are not actual traffic problems, like 'slow traffic' during rush hour in the city. It's going to be slow all over during rush hour so there's no point to rerouting.