If you're entering or leaving Istanbul by car, E80 runs from the Bulgarian border and through Turkish Thrace to Istanbul, continuing on to central Anatolia in the east; this toll road is the best of several alternatives. Getting out of the city by car can be challenging, as the signs aren't always clear. It's always useful to have a driving map.
Istanbul is notorious for congested traffic, a cavalier attitude to traffic regulations, poor signposting, and a shortage of parking spaces. In short, don't even think about renting a car for travel in the city.