I am not sure Carta has a bus going to Summerville anymore. They tried a few years ago to do an express bus from downtown all the way to Summerville via Dorchester but I heard somewhere along the way there were not enough riders to keep it profitable. The phone number for Carta is (843) 747-0922. You can call them to check for sure. I do know there is a bus for upper Dorchester but it only goes near the Air Force base and crosses over on Ashley Phosphate to the N Charleston area of Rivers Ave.
Carpooling may be the better option and there are websites where you can make requests to match up with people wanting the share gas expense and ride together. Try https://www.carpoolworld.com
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