Cities described as overwhelmed by tourism are usually empty a few blocks from the pressure points. The crowding is intensely local, and its causes are physical.
Attention concentrates before people do
Visitors arrive with a short list assembled from the same sources: the same handful of sights, viewpoints and photographs.
That list is far shorter than the number of interesting places in any large city, so demand piles onto a small set of coordinates.
Recommendation systems reinforce it. A place that is already popular ranks higher in search results and social feeds, which brings more visitors, more photographs and a further rise in ranking.
The effect is that attention concentrates faster than visitor numbers grow, so a city can see modest growth in arrivals while a single square becomes unbearable.
Old street geometry cannot absorb it
The districts that attract visitors are usually the oldest, which means narrow lanes, small squares and buildings that cannot be widened.
Pedestrian capacity is a function of width, and once flow exceeds it, movement slows and the crowd deepens rather than spreading out.
A single bottleneck, such as a bridge or an alley, can hold a queue that fills the streets behind it for hours.
Arrival timing stacks the peak
Cruise calls, tour coaches and day trains deliver visitors in blocks rather than a steady stream, and those blocks often align.
Day visitors also follow near-identical routes, because a fixed departure time forces a route that ends back at the port or station.
The result is a few hours of severe pressure followed by quiet streets, which makes daily visitor totals a poor measure of the problem.
Residents leave the affected blocks
Short-term rentals pay more per night than long tenancies yield per month, so property near the pressure points converts.
Ordinary retail follows the residents out and is replaced by businesses serving visitors, which removes the reason for locals to enter the area.
Once residents have gone the district loses the daily life that made it worth visiting, which is the self-defeating part of the process.
Measures target the peak, not the total
Cities that manage this well work on distribution rather than volume: timed entry, staggered cruise berths, and promoting alternative districts and seasons.
Day-visitor charges and coach restrictions attack the same peak from the arrival side.
What consistently fails is asking visitors to be considerate, because the crowding is produced by where everyone is standing rather than how they are behaving.