My car has an indicator that the engine is cold when I turn it on in the mornings - the weather is below freezing in my area. Do I need to warm the car before using it or can that warning be safely ignored? As a follow-up - if it is okay to drive with the light on should I avoid the highways until it goes off and just stick to slower speeds?

I'd like to avoid the waste that goes with idling the car every day if possible.