(https://drive.google.com/uc?id=1M7A0eVG0zrActi-MMTMvVKVrkFnDizF-)
The above graph would be strictly more informative if it showed the actual number of city cars in the city, and thus whether they are increasing or decreasing. The attached save appears to have a significant number of them and they are affecting trip times.
The problem is that the current number of city car within the city is very hard to track, but citycars are spawn from the city, so tracking them is easy. One could register cars to a city when they ecnounter a city road, but how would that work if there are overlapping citys? And this would imply quite some overhead just for the statistics.
Isn't how many city cars spawned by a city indicative of something as well, which would be lost if the graph reports something else? Since they move around at random once spawned, how many of the are in a city isn't really indicative of anything game related.
If the effect on trip times is of interest, I would think a report on the total number of road vehicles would be more useful. I find the efficiency of my routes are more affected by vehicles from my other routes than by city cars. And most of the big problems city cars have caused have been the work of a single city car getting into a bad spot, usually inside a freight yard. Not to mention the bad effect sheep can have on rural routes.