Avoid Rush hours (08:00-10:00, 16:00-19:00)
Travel during off peak times. You WONT get a taxi during rush hour, and if you do, you will just be sitting in a car wishing you hadn't
If you absolutely MUST travel during rush hour, the metro system is the way to do it (BTS, MRT, BRT), but it will be very busy.
BTS / MRT
They are not 24 hours, but typically run 05:00ish to 23:30ish. Don't trust google maps time tables for the start and end times, during the day they are fine. Sometimes the start and end times are uncertain, best to ask the staff if its important
You will save a bunch of time getting a BTS/MRT/Rabbit card, bring you passport to the kiosks at every station and prepay for a card. at the end of your trip you can refund anything left over (you will need your passport)
You can buy one time tickets using cash, coins, or QR codes (with a Thai banking app). If you have large bills you will need to go to the kiosk. The staff at the kiosks are nice and will help you(unless its rush hour, then they are nice, but busy).
When you get do your destination station, you can use a motorbike taxi, or a tuktuk to finish your last leg. I recommend using Movmi.co, a ridesharing tuktuk that operates at most of the bts stations, you will need a app.
BTS is above ground train stations which are not air conditioned, but the trains are air conditioned.
MRT is below ground train stations, trains are air conditioned.
BRT is ground level "bus" stations, the busses are air conditioned, and have "semi" dedicated traffic lanes.
MRT Payment
If you have a contactless credit card (Visa, mastercard, Europay), you can tap to pay on the MRT (not the BTS, or BRT).
Rain
If its raining, or projected to rain, that doubles the rush hour effects, plan ahead. You can pick up umbrellas at most combinis
Taxi Apps (Grab, Bolt)
These work really well, you don't have to communicate at all, and you get a fixed price. You will have to wait 5-10m for the taxi to arrive.
Hailing Taxis on the street
This is hard mode, have your destination loaded up on your maps ahead of time. Flag down a taxi driving by (point your hand down at a 45 degree angle, and fan the ground). Many (most?) of the taxi's will refuse to turn the meter on and will want to negotiate a outrageous price, just be polite and ask for the meter, if they say no, just get out. (hopefully this has changed, but I only use the Apps because I hate haggling). Insist the meter is on before you drive away in the taxi.
There is a fleet of taxi's called "Meter Taxi" as the company name, so when you insisted on the "Meter" the drivers would say yes, METER Taxi! but would refuse to turn on the meter... It was a hilarious Kafkaesque Abbot and Costello routine.
Showing the driver your destination on a map will make things easier, especially if your going down some complicated Soi.
Airport/ARL/Airport Rail Link
From the BKK airport there is a air-rail link connecting to the BTS system, it works pretty well, but you will have the buy a separate ticket. Taxi's are faster, unless its rush hour. The train is air conditioned, and there are elevators throughout the whole system.
Airport - Inside
The BKK airport is very large, if you want you can pay $50 for a service that will pick you up from your flight in a golf cart, and drive you to a special immigration queue "fast trak". If you have carry on only luggage it can be worth it. Beats standing around in queues for a hour.
Many of the airport departure lounges offer showers, so if you have lounge access (priority pass, airline loyalty, or even out of pocket day passes), you can plan on taking a shower at the airport before your flight.
Asking for help
If its in a well lit area, and people are not clearly busy, its ok to ask for help. Use simple English, spoken clearly and slowly. Most people have taken English classes, but they may not be fluent.
Hi! Can you help me? How should the motorobike taxi charge to go to X?
Hello! Do you know how much I should pay the TukTuk?
People are very friendly, if they are not busy. Don't be demanding or pushy. If someone isn't responding to you, or just kinda shakes their head. Say thank you, or no problem and move on.
Q&A
Any other questions, or ideas, just ask, happy to help!
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Line Man
You need to have Line installed with a Thai phone number, but you can order Motorbike taxi's to pick you up and drop you off from anywhere.