Most of Ontario Northland's Toronto bus services terminate at Yorkdale without continuing to Union. Because it's faster to take the subway from Hwy 407 station than it would be to stay on the bus anyway.
Here's the current schedule:
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My proposal is to simply swap Yorkdale for Pearson. It only takes 5 minutes longer to get from Hwy 407 Station to Pearson than it does to Yorkdale, so it would be a pretty minimal change to most ONTC schedules. For the few trips that do actually go to Union, they could either truncate the trips at Pearson or add about 10 minutes to their trip to Union. Getting from Pearson to Union doesn't take much longer than getting from Yorkdale to Union anyway.
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