There is a funny little by-products of all this. If you are going on a trip say from Chiang Mai to Bangkok and you don't feel like adhering to speeding rules, you can actually go to your local police station and pre-pay a speeding ticket and this gives you the "right" to drive as fast as you want to for 3 days.
People very rarely get done for speeding in this country anyway, but there are some speed traps and if you had the foresight to pre-pay this offense, you just leave the scene... no problem! And we wonder why the road death rate is routinely in the top 3 for Thailand.