Ever driven to a big sporting event?  If so, you'd see a perfect example of just how wrong your claim is.  Notice where the traffic is worst... right around the stadium where the vehicles are more densely compacted on the roads.  But when those same vehicles get a few miles away from the stadium, traffic is better because they spread out over more streets and a wider area.  

That's the effect of HIGH DENSITY.

Building a 1200-unit high rise apartment, even if it's next to a rail station, is going to INCREASE the number of cars on the road in that immediate area (relative to not have 1200 apartments in that area).  That's painfully obvious.