I don't see a need for the law to specify a time. You have to stop and you proceed as soon as you have the right of way and it is safe to do so.
For a working AI, it can still stop safely if the pedestrian starts moving into the street in the few seconds the car is crossing.
Computers do exactly what they are told to do and never do what you did not tell them to do. They do not have any common sense that you did not give them. As you build more and more capable computers and give them more influence and control, you damn well better have an exhaustive, complete and comprehensive set of values for it to follow.
At first this will be due to AI's one current superhuman ability: speed; a classic is the do you drive into the old couple or the little kid car scenario. It's not really an issue for humans because the situation is over before we can make more than a reflexive response, but the AI can take its time to make a concrete decision. This isn't a thing we spend a lot of time thinking or blogging about and a societal consensus probably has not formed and wouldn't show up in the data. Presumably the AI would pick at more or less random as we are forced to do, but I can see an entity capable of superspeed choosing to perform random actions as causing other as yet unforseen problems.
The big pinch is if and when we create transformational AI. We will be presented with options we had never imagined being possible. Do you keep Mars as a memory of the old days, or do you use its crust to construct interstellar spaceships and build better lives for Earth humans? Do you allow humans to live lives as they do now, or do you modify their biology and neurology to live longer lives and never engage in antisocial behavior?
These are lazy obvious examples that I came up with but there will be many less-cosmic decisions we will be presented with that we absolutely do not have an existing moral template to guide us.
How many seconds is one supposed to stop at a stop sign? The law is silent on the matter. There is so much around laws or rule systems that forms as conventions around the rules that are often just as important as the text of the rules themselves.
See also solid white lines (do not cross) in the US. People cross them for pragmatic reasons in many locations all the time, often in full view of law enforcement.
There are also many road users for whom laws are almost never enforced and often disobeyed, such as cyclists. In many places cyclists are bound to obey the same traffic lights as motor vehicles. This plainly doesn't happen 100 percent of the time, and again is often tolerated by law enforcement. A car with an absolutist interpretation of driving regulations would therefore still have to deal with the fact cyclists and others may not share in this view.