In some cases, it may be due to the accumulation of dust and dirt.
However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges.
After rainfall, the bridge surface absorbs water and retains it for a certain period of time.
Hence, this provides a good habitat for fungus or algae to grow.
Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth.
Improvement in drainage details an