From https://itsbetterinthailand.com/how-to-use-local-taxis-in-bangkok-without-getting-ripped-off-or-lost/ (mirror 1, mirror 2):
If it’s after midnight, early hours of the morning or a weekend, it’s likely you don’t need to use the tollways.
Why is it unlikely one needs to use the tollways when riding a taxi after midnight, early hours of the morning or a weekend in Bangkok?
E.g. is that because there is less traffic at these times and subsequently the use of the roads with tollways is less interesting, or is that because there is fewer roads with active tollways at these times?