I've been on that route once, from BKK to the Marriott Executive Apts Sathorn, which isn't far from the Banyan Tree, if you were on Highway 9. When it seemed my driver was in the wrong lane on the usual route I was ready to explode until I looked down at and saw my GPS was agreeing. The driver also had his GPS up. Apparently there was a red warning on the usual route, so we likely saved time. If you scroll up you will see I'm not someone who often likes cabbie detours, but this one can be legit. It's not that far out of the way and keeps you away from the Sukhumvit area traffic.

This is why I always program my GPS when I get in a taxi.