Source: https://www.dreamstime.com/public-stop-design-concept-isometric-city-public-transport-stop-design-concept-text-captions-images-vehicles-image190808632 (November 2021)
Diagram of passenger travel time as a function of stop density
Source: Štefančić, G.: Urban Transport Technology II, University of Zagreb Faculty of Transport and Traffic Sciences, Zagreb, 2010.
Tp – passenger travel time (travel/hour)
tv- travel time
tpj – walking from and to the stop
g* – optimal stop density for minimum travel time
g – stop density (stop/kilometer)
Assuming that the number of passengers on a line is constant (meaning there are no transfers) an increase in stop density results in a decrease in the average walking distance and waiting time for the mean of transport. Driving time increases due to stopping at stops. Initially, the increase is linear (each stop adds a fixed delay time). When the distances between stops become so short that the means of transport cannot reach the maximum permitted driving speed, the additional delay per stop increases, thus increasing the driving time and thus the total travel time.
Diagram of walking distance by type of public transport service
Source: Štefančić, G.: Urban Transport Technology II, University of Zagreb Faculty of Transport and Traffic Sciences, Zagreb, 2010. 
A – walking to street public transport
B – walking to metro
C – walking to regional railway