score:8
I did my driving test in Iran 10 years ago and I don't remember quite well this sign but as far as I know and based on my experience, the car which is ahead has the priority to the one that is behind.
Edit:
According to what you said as the situation, once again I'll confirm my reasoning that the front car did not make a mistake. In Iran, driving rules are in line with international standards so the vehicle that hits the vehicle ahead is almost always 100% responsible. Even if the vehicle ahead stops, abruptly, or slows down suddenly, the onus is on the vehicle traveling behind to keep a safe distance (everywhere even in that situation you explained).