Usually, we expect that our foreigner guests are the punctual person. So act according to that expectation. In calculating travelling time in Bangkok you need a local expert. We have a lot of new construction that might impact the normal routing time that you never know. Ask local people of any new constructions like Bridges, High Speed Trains, or other road digging(for telephone line or water work). Ask them about upcoming major events like Royal Ceremonies, because sometimes the road is blocked for an hour or more. One last question is about the recent changes in traffic flow. Remind your local counter part about these issues. Many meetings do start late because the Thais who are suppose to pick up the foreigners are late(even if you are not!).
Please be patient if Thai customer's do late. They expect you to come on time. However, they might be slightly late from five minutes to half an hour. Ask for his or her mobile phone number, so you can check and estmate arrival time when they have not shown up yet