Speed averages between trail riding and road riding

I've been riding for a couple years now. I started out on trails/tracks and
finally got bored enough with those that I began venturing out on the road.
Since that time, I've noticed my averages went up considerably. I think
back, and the only thing I can wonder is whether it has to do with having
cars all around me to keep me pushing harder and harder to avoid being hit.
When I start at lights, I end up drag racing the cars next to me...and at
times win (at least until we get to the other side of the intersection)!
While riding, I notice that I'm trying to keep up with the traffic around
me. I've even noticed when I'm out riding late at night when the traffic
isn't out, I tend to ease off the hard riding. All of this has pushed me
into a new level of riding experience and overall into a higher bracket of
times and speed averages.

Has anyone else ever noticed the differences like this when going from
trails and into traffic road riding? Does anyone have opinions to whether
it's better to get into heavy traffic or just isolated traffic?