what an amazing drive the Trans-Canada Highway is but good luck trying to find the starting (or ending) point of it. As far as I know, the Trans-Canada Highway begins at an unmarked street corner in St. John’s in Newfoundland & Labrador. One of the highlights of this drive is driving along Confederation Bridge from New Brunswick over to Prince Edward Island.
to go across the bridge to PEI I have done the crossing on the ferry car and driving across the bridge. Taking the car on the ferry is more expensive than driving along the bridge. I think the drive along the bridge is $45 for a passenger car, and the ferry is $68 for a passenger car.
Re: to go across the bridge to PEI if you take the ferry to PEI and want to drive back across the bridge back, be aware that when you take the ferry one-way to PEI, you can't make a reservation, and cars with reservations get priority. So if it's a busy time, you might not make it onto the boat.