Traffic on Overlea Blvd and Don Mills Road suffers from a funnel effect,... traffic converge to use these major arteries to cross the valley or use the DVP.
BTW, take a look at the traffic congestion throughout Toronto, the majority of Toronto would "fail a traffic study"!
A costco is the lesser of 2 evils. The area is already a shopping district with the East York Town Centre mall across the street. By adding more retail space (ie Costco) instead of condo or offices, the traffic that's mainly added is not added during peak time of AM or PM rush hour. Whereas, condos and offices would mainly add to AM and PM rush hour traffic congestion.