Yes, stop light is used as a general term for traffic light. Likewise in the states we do not use the plural of lights, unless when referring to multiple traffic lights. From past comments it seems only the American version was accepted but British is as well now.
I've been curious if this was a regional thing and how far spread it might be - town? State? I asked someone from a different major US city from here in the Northeast what they call it and they replied "stop light" as well, so it is at least a regional thing. I also Googled "stop light" and got search results for "traffic light" just fine (no Did you mean: suggestion, although I know this probably isn't the most definitive use XD )
I could argue that traffic lights should not be accepted because that is too specific since it is plural and semafor is singular. If it was traffic lights then DL would probably have given semaforuri or semafoare ;-)
However, it isn't always about vocabulary with these exercises. There are plenty of exercises here where you do not have literal translations. For example, in Romanian, as well as Italian and Spanish (and I can only presume other Romance languages) you refer to the number of years a person has and not the actual years. It is more possessive (Eu am treizeci de ani/"I have thirty [of] years) whereas in English we focus on being an age ("I am thirty years [old]"/Eu sunt treizeci ani [veche]). Romanian also requires the use of the preposition de in some cases, which is not necessary in English.
Likewise some words do not even have English equivalents such as dor, doină, and colindă; as given in the Tips and Notes section.
Perhaps "stop" is not a literal translation, but it isn't necessarily too specific (unless where you are you have things that are specifically stop lights which are ubiquitous and completely different from traffic lights). Perhaps humorously (or to translate for the British English speakers, houmourously :-P ) if you want to think about why it is called a stop light in some regions it is because it is used to stop traffic. It doesn't make sense to call it a go light because why would you need to tell traffic when to go? You are already supposed to be going on roads, and if you aren't moving on a road then you are in a parking lot and thus you would not need a traffic signal. It is a very British thing of us to call something by the literal thing it does (such as calling an elevator a "lift" :-P )
Also, I do not know if "traffic signal" is an accepted answer (I just recently reached Level 5 for this and I haven't practiced it), but that should be an acceptable answer as well, if it already isn't. Does this also mean "traffic signal s " should be added as a correct translation?
Joe, Many thanks. I think in England we use lights because each set actually does have lights in the plural - although it could equally be said that there are usually several sets so that all drivers can see and a single bulb failure (before LED days) is no excuse. My impression (less exposure) is that we can also use singular or plural for rail signals. I stopped the train at the signal if fine, but I stopped the train at the signals is also fine.
As you say differences in linguistic patterns and literal translation won't do.
Of course they are called traffic lights in UK because they have been put there by decision of Government as lights to control the flow of traffic; the stop, go sequence is controlled often by a computer programme that makes sure traffic moves efficiently across a whole area. The sequence is red for stop, red and amber for prepare to go but do not go yet, green for go, amber for prepare to stop, back to red again for stop. The three lights are in vertical column. Not sure what Romanian traffic lights look like or do...?