This photo is from their Facebook page. The photo caption reads (from Google Translate):
So many cars disappear from the streets of our cities, when their owners will fill one bus. Public transportation! This number of cars would be unnecessary, if Their owners changed to a bus. Public transport!
It looks like the bus in the photo is Solaris Urbino 18 Hybrid, which is one of the manufacturer's newest products, and is a very modern hybrid vehicle. Based on the product brochure, it the densest configuration, it can carry 161(!) passengers, which is quite impressive. It looks like there are about thirty rows of private cars, five cars in a row, so let's make a wild guess and assume that there are 161 private cars on the left side of the picture. The point is clear: many private cars actually carry only one passenger, the driver, and when you fully utilize a bus, you can carry lots of people.
The good bus
How many of the 161 passengers - who now have abandoned their cars and only commute by bus - can actually have a seat? Answer: 51. Assuming that all passengers are average-sized and able-bodied, 110 of them will have to stand for the whole journey, and they cannot take any wheelchairs or a baby strollers.
It is true that number of vehicles on the right (1) is less than the number of vehicles on the left (161). The bus full of people would be much more efficient on transporting the people than the cars are, which is the point of mass transportation in general.
These publicity campaigns consistently compare the "best case" scenario for buses and "worst case" for cars, and this may lead to unrealistic expectations. In the scenario of the image, the bus will be as full as it can legally be. It probably won't be comfortable, fast or maybe not even safer than private car (there are no safety belts or air bags on buses, except for the driver). Because it is full, stops will be long and cumbersome, some people might even have to step out to let other passengers out, and then board again. The bus will also use slower and smaller roads to pick up passengers, and will usually not take the passengers to their ultimate destination - they will have to switch to another bus or walk (or use other form of transportation).
The mass transportation model works well in crowded cities for commuting, but actual occupancy rates are often much lower. According to UK Department for Transport's report, the estimated average occupancy rate (vehicle miles divided by passenger miles) of buses in London was 19.3, which is good - but only 9.1 outside of London (11.1 for the whole Great Britain). It also mentions that the current trend is that the rate for outside London is falling, potentially making mass transportation less effective and more challenging to maintain.
The bad private cars
According to University of Michigan Center for Sustainable Systems' report, average occupancy rate of all U.S. vehicles was 1.55 in 2011. The average rate varies by trip purpose - for leisure it is close to 2, for commuting it is often close to 1, and sometimes the car can also be full(!).
1.55 passengers is 31 % of a passenger vehicles maximum capacity (assumed five). 11.1 passengers is only 15.9 % of maximum capacity of a traditional city bus (40 seats and 30 standing, rough estimate).
With these figures, you only need two or three cars to match the average occupancy rate of a bus, and with a car you can travel from point to point with greater flexibility. Fuel efficiency for new cars is more than 25 miles per gallon (sorry for using non-standard units!) and for an ordinary U.S. city bus, according to National Renewal Energy Laboratory's study, was roughly 4.0 mpg (Appendix C).
With 11 passengers, this would equal to 44.0 passenger miles per gallon (we're talking approximations here, in reality of course fuel economy changes with the load), which is only slightly better than for a modern passenger vehicle with only 1 occupant! A full passenger vehicle would clock near 150 passenger miles per gallon (assuming no extra hurdles, heavy traffic, stops etc.) - and even with average 1.55 passengers it would still clock an impressive 38.75 pmpg.
Based on these numbers, driving a passenger car alone is approximately as fuel efficient as it is to travel on a bus, and with at least two passenger, it is significantly more efficient. At the same time, it is much more comfortable, flexible, faster and safer, and it allows you to transport groceries, strollers and other items.
However, this is not the complete picture either, for many reasons.
While that 4.0 mpg for a city bus is measured based on actual vehicle miles and fuel consumption, directly comparing it to passenger car fuel deficiency is difficult. Cars are used for much more than commuting, and often buses simply do not go where drivers would like to go. Often bus trip involves somebody driving a private car to pick up a passenger, because the nearest bus stop is too far away (or in a dangerous place) to walk. With newer buses (and hybrid and electric engines) efficiency can be improved - and bus is not the only form of mass transportation in many cities.
Good city and transport planning can improve public transport by planning the city around it and making it easy to catch a bus or train and switch between them. However, it may create dense suburbs without which heavy rail transport might not be worth building there.
And, perhaps most importantly, the average occupancy rate can be vastly improved. If people really would ride the bus when it is practically possible, there could be less traffic on the roads, less pollution and better fuel economy (per passenger) for buses. Owners (and drivers) of private cars are not (all) stupid. Many of them would ride the bus, subway, train or tram if it was feasible, but it is not because of bad route planning. I was unfortunate to experience this myself in 2009 when I started in a new job which was located only about 5 km from my home - but travel time there by bus was almost 40 minutes, and involved one or two switches. I bought a car (Citroën C1, 64.0 mpg) and haven't regretted that decision one second. (I still walked there sometimes.)
Frank Palmer of Daily Kos wrote about the "mpg fallacy" - and made some of the same points I make here.
We will not and practically cannot achieve the utopian future where private cars have been exchanged for mass transportation - at least not in near future, and meanwhile, a more complete picture of different forms of commuting and transportation should be discussed in public. Mass transit is good, but by average not nearly as good as it is claimed to be. Sadly.
P.S. The modern hybrid bus in the image would achieve around 1000 pmpg.