Let's start off by saying that English probably has more exceptions than rules. Keeping that in mind: Uncountable nouns do not usually have a plural form. Road has "roads", so it is countable. There is no plural for Traffic, so it is not countable.
(It's absolutely fine if you want to stop reading at this point to avoid confusion. But to be thorough, I include the following.)
As for exceptions, some uncountable words have plural forms. Water is uncountable, but you can sail uncharted waters. The thing is, those plural forms are usually rare and confined to a specific definition. "Waters" in this sense would refer to seas and oceans. (You might also hear people order "two waters" to drink. That's just a lazy way of saying two bottles or cups of water.)
Some countable nouns can also be uncountable in context. I am going to New York by train. Train is usually a countable noun, but in this case it is being used as an abstract method of travel, not a particular vehicle.
But if you ignore those exceptions and just follow the general rule I said at the beginning, you will almost always be correct.