If pollution levels spike above a certain level on two consecutive days, the most polluting vehicles – which have to carry special 'environmental plates' are forbidden from entering the city.
The Qashqai's satnav suggested travelling through Serbia made for the quickest route to Sofia, our halt for Saturday night. So we made quick progress south through the grey, flat expanses of Hungary to the border.
The Serbian border officials waved us through quickly (one was making a phone call, the other was about to tuck into his lunch, so we quickly realised where we were on their priority list).
Shock number one was the drop in the quality of the road surfaces, although they improved dramatically as we approached major population centres. Another surprise were the regular signs warning us of upcoming toll booths – only for us to discover that many of the booths hadn't yet been built.
Our first stop was Novi Sad, Serbia's second city behind Belgrade. We were interested to note that Novi Sad is twinned with both Norwich and Modena – two cities with keen car making cultures. Seemed like a good omen for our stop there
We passed a bustling open-air Saturday afternoon market and negotiated our first horse and cart of the trip on our route to the modern part of the city. Then there was a real treat; we crossed the broad Danube river and stumbled upon the old part of the city.
We stopped for coffee at the Petrovaradin Fortress, the second oldest fortress in the world. Although most of the walls are long-since overgrown, you can still trace the clear outlines of the fortifications. Better still, you can drive right to the top and imagine the various waves of invaders the place had resisted, dating way back to the Bronze Age.
Pressing on, we briefly stopped in Belgrade. Snapper Stan, who has an eye for these things, noted the daunting abundance of stark grey concrete buildings, as if the communist town planners had really sunk their teeth into the city. Nevertheless, Belgrade still has some achingly pretty areas, not least Kalemegdan Park on a hill overlooking the city and the Park of Friendship on the opposite side of the Sava river. It was sad to see some adventurous architecture, such as the striking Museum of Contemporary Art down by the river, fenced off from public enjoyment and seemingly decaying.
We headed south east, making prodigious use of the Qashqai's cruise control function until the motorway ran out near to the Serbia-Bulgaria border. Both Stan and I forgot to remember that we'd shuffled across another time zone, which meant we didn't reach our stopover for the night until well past 10pm.
The outskirts of Sofia has some terribly uneven roads layered with shiny tar, making it tremendously difficult to see road markings when it has been raining, as it had yesterday. I think I spent most of the journey into the city inadvertently straddling two lanes, although I wasn't alone in this so I didn't feel too guilty.
Today we've got approximately 300 miles to cover to reach Istanbul, the point where Europe collides with Asia. That fusion of East meets West is responsible for creating the vehicle we're driving on this mammoth journey, so I'm interested to see what life is like on the banks of the Bosphorus.
Follow our progress at @matt_burt_ and @autocar.