The tread on a road bike's tires is really quite unimportant and purely cosmetic.
Road bike tires have tread patterns because they sell better, not because they perform better.
Here's some questions to ask to decide about replacing a tire:
Can you see any of the fabric?
Are you getting flats more easily than you used to?
Is any part of the tire bulging out or lumpy?
Are there visible holes/gashes in the tire?
Does the wear indicator indicate you should replace?
(Some tires have dimples or grooves that are there to indicate when the tire should be replaced.
Some have an alternate color of rubber underneath, and when that shows through the tire should be replaced)
Smooth tread is actually ideal for road conditions, but not commonly found on tires.
If you're on hard surfaces such as pavement/asphalt, the tread doesn't help you and actually slightly reduces contact area and increases rolling resistance.
On most road tires the minimal tread pattern is just cosmetic to make their customers feel better (since a smooth tire looks slippery).
See also:
Sheldon Brown's comments on tire replacement and tread patterns If you plan to ride through mud, sand or other soft surfaces, you need tread.
So when you no longer have tread on your mountain bike or cyclocross bike's tires, it's time to replace them (unless you plan to only use them on the road).
bicycles.stackexchange.com/users/76/freiheit
Many road tyres would have wear indicator .
Usually that would be a small hollow on the tread.
As long as it is visible the tyre is ok, once it is gone, the tyre should be replaced as that indicates that the main rubber layer is worn out, the grip is no longer that good
and it may not be safe to ride.
bicycles.stackexchange.com/users/83/kristof
For the last two worn-out road tires I replaced, I've noticed a pattern in wear that seems to be fairly consistent.
This isn't a large sample size, but the tires were from different brands, and the wear seemed consistent with various comments I've seen from others.
Based on your own tradeoff between cost- and safety-consciousness, you have to decide at which stage in the wear process you need to replace the tire.
First, the tread (if present) begins to wear.
Not all road tires have tread, and in most cases it's purely cosmetic.
Then, the tire develops a flattened, almost squared-off shape instead of the round shape that a new tire has.
Next, the tire starts getting a lot of small cuts in a short period of time.
This doesn't necessarily mean there are flats or that the cuts are deep.
(After removing the tire, I could more easily see that the outermost layer of rubber was extremely thin and beginning to delaminate.
This explains why I was seeing so many small cuts.)
Finally, the outermost layer of rubber wears through and threads begin to appear.
By this point, the tire is definitely unsafe.
This process might not be exactly the same for all tire models, but I've noticed it in several of them.
Of course, even a new tire should be replaced if it develops any signs of trauma as discussed in other answers (bulges, cracks, or gashes), or if the frequency of punctures increases.
bicycles.stackexchange.com/users/3649/amcnabb
I replace my tyres when i start to get a lot of punctures.
With new tyres, i rarely get punctures, despite what i see that i cycle over.
after some time, and wear, i suddenly start getting a lot of punctures.
As a hypothetical that doesnt happen: If i dont get any punctures and i start to see the tyre fibers coming through, its definiely time to replace the tyre.
I know it meets no manufacturers or performance guidelines, but its a method that requires absolutely no monitoring and lets me get all the life i can out of the tyre.
bicycles.stackexchange.com/users/10999/497362
Rubber degrades over time due to ultraviolet light damage and oxidation.
Old tires where the rubber looks dried out with lots of cracking or rubber coming away from the fabric carcass should be replaced.
bicycles.stackexchange.com/users/24228/Argenti Apparatus
The treads on my road bike's 28C tires are almost completely gone—they only persist along the shoulders of the tire.  Do the treads matter?  What is a good indicator of when the tire as a whole needs to be replaced?
How do I know when to replace my tires?
bicycles.stackexchange.com
<tire><road-bike>
bicycles.stackexchange.com/questions/437
bicycles.stackexchange.com/users/17/Drew Stephens
Tread is not a reliable indicator of wear on road bikes as it is for purely cosmetic purposes. On mountain bikes, however, once the tread is gone, it is time to replace them. On road bikes look to signs of wear or change tire if punctures are happening frequently. Some people argue that worn treads mean you should replace your tires. However, treads are not necessarily a decisive indicator, as they are often cosmetic and many models have discarded them. Regardless, it is recommended that you change your tires if they have signs of wear or damage, including frequent punctures. You could also advise your tires' wear indicator, if any.
Some people argue that worn treads mean you should replace your tires. However, treads are not necessarily a decisive indicator, as they are often cosmetic and many models have discarded them. It is recommended that you change your tires if they have signs of wear or damage, including frequent punctures. You could advise your tires' wear indicator, if any.