How do you deal with that ?
The car...well...only getting slow down. One way or another eventually it will crash into the gates (i use the typical safe crossing railway circuit).

And i made a simple test map where the car just looping around the track to see if the problem will occur in (simulated) heavy train traffic, and it did,..a lot of times.
It keep crashing the gates.

I can't set the speed limit to zero before crossing; based on upcoming train signal, or the gates status can i?
if so, how the car will moving again ? Since it need to pass "remove speed limit sign" to remove the impose speed limit?

I can't set it to very low either, say 5 km/h before crossing, to ensure there are enough time to slow down.
It's just too slow. And it's not bulletproof setup either, the rate of crashing is just lower than with higher speed limit.

OK, maybe it's just me not being smart enough to figured out this.
So,,...how ?

This is actually not covered yet. What you want is some sort of "stop sign", right? I thought about something like this, but never actually implemented it. It would go as following:
- There is a new "stop" sign
- If the stop sign is "active", i.e. some kind of circuit signal is > 0, any vehicle driving across it will be slowed down to 0 km/h.
- The vehicle "remembers" wich stop signal it ordered to halt
- If this particular stop signal is switched off, the vehicle is automatically started, resuming it's travel

Something like this?

How about a dual functionality of the current sign, speed limit sign can work both way; to make the vehicle go, even when the car is stopping.
I mean, any positive speed value is basically a player want the vehicle move forward at that speed.
So having a functionality to make the vehicle goes forward automatically would not making it conflicting with what player want to achieve, does it ?
They will go forward in any case.

With negative value, it's even better, so let's say a player want to park a car, but miss the spot and hit the wall garage.
But next on that wall garage there speed limit sign with negative number, it will tell the car to move backwards.

Then the car will move backwards until it hit the sign with zero value (or 1, since zero in factorio means no signal ?)
It would be neat since with correct layout, the vehicle can auto refuel-refill ammo.

Anyway, the negative value speed in factorio means the vehicle move backwards, it's just makes more sense.
To prevent it from being locked due zero speed limit, a manual control can always override the impose speed limit.

Thus that would remove the need for a new entity that do "stop and go"
And it will be less confusing (well, at least for me)

Now, for rail crossing, if the current sign can make the car go automatically, i can do :

• At 9 chunk away, i put 100 km/h, to ensure vehicle have the time to brake, if i enter the crossing at high speed - (260km/h), and there's incoming train.
• At 2 chunk away, i put 50 km/h to further ensure i have the time to brake, or to increase speed if i start too slow, to prevent overbrake.
• At 1 chunk, 25km/h to ensure it will not underbrake.
• At 1 tiles from gates i put a sign with zero value.
  All of those will be circuit controlled.

Then all i have to do is waiting for green signal from rail/gates, output as positive value+arithmatic combi to make my vehicle goes forward automatically.

Btw, i just test it, a zero value in speed limit will not do anything, but setting it to 1 will make the vehicle stop (a bug ? Or is it intended)

For existing player that already park their car in the speed limit sign, yeah it will make their car move forward when they enter it with auto-cruise enabled.
In a way, yeah it will break their setup.
But it's one time deal, all they had to do is adjust the sign accordingly to new rules.

I'm currently trying to build a rail crossing and realizing what I need is essentially just a traffic light that can show green/red, controlled by the circuit network. Green means go at normal speed (remove all speed limits), red means stop (speed limit of zero). Any chance a feature like this will get added? It seems essential if the purpose of this mod is to be able to design a rudimentary road traffic system.

Maybe, the same way you check the road tiles / signs, you could check the gate state? e.g. If a car sees a blocked gate, it stops and waits for the green signal. Not sure if it could be done, if it works well with parallel to the road gates, and if it would be easily compatible with modded gates, just leaving my thoughts.
If it's impossible, then my thought is that @Arcitos's idea with "Stop" sign is more appropriate for such situation, furthermore, you can make cars drop speed for "limit" sign, and force it to brake for "Stop" sign.

Hi, the latest 0.18 update features a new stop sign and a road sensor which can be uses to create automated railroad crossings. See the FAQ for a brief explanation.

Thank you again for your contributions and ideas, as this discussion proved crucial for the development of the new features!