"Therefore there should be some 70kmph speed distances on the way too. But if you concentrate on the 50kmph speed limits shown throughout, it is logically correct to assume the overall speed limit for the whole way from Colombo to Negombo is just 50kmph, as there are no 70kmph signs. "
look out for the 50 sign with a line crossing it indicating that you are now no longer required to maintain 50 but can go upto 72. I have heard cops having speed traps just before this sign so that any argument the motorist brings can be promptly shot down by just pointing at the sign