Chalong Municipality has installed a sign warning motorists of one section 35 metres long that has collapsed into the water, some 10 metres below.
The danger area has been marked off with traffic cones and old tyres, for safety.
Local residents pointed out that many sections of the road that circles the reservoir, located behind Luang Pu Supha Temple, have already fallen into the water, which has not risen.
Some sections of the road show cracks up to 100m long, they pointed out.
If no action is taken, it is only a matter of time before more large sections of the road fall into the water, they said.
“Local villagers have had accidents many times because the road around the reservoir has collapsed,” said local resident Wichian Sangsai, 53.
“The responsible agency has come to inspect [the damage], but have not come to make any repairs or improvements at all,” he added.