"Why Count Buses? Probably because they're recognizable and regular"

On the first and last days of this book, Perec tells us about most if not all buses that he sees pass by (on Saturday, he skips this because he's "lost all interest in them"). He also regularly tells us the time, so we can make estimates about the regularity of 5 of the 6 bus lines that run through the place (we must ignore the 84 because when he is sitting in the Café de la Marie Perec notes that "it is only by chance that [he] can see 84s pass by").

On Friday, Perec counts buses over a 390 minute period (non-contiguous, there are some breaks and he is unable to see any buses from his location at la Fontaine Saint-Sulpice). His bus tally is: 63: 31 buses 70: 26 buses 86: 28 buses 87: 20 buses 96: 30 buses

Paris maintains the same bus numbers for the 5 remaining lines so we can also compare the schedule RATP currently lists on their website (dated at November 2019). For several lines, frequency changes by time of day, so min and max intervals will be given. 63: 12.5' (1975) vs. 6-8' (2019) 70: 15' (1975) vs. 8-10' (2019) 86: 14' (1975) vs. 11-16' (2019) 87: 19.5' (1975) vs. 10-12' (2019) 96: 13' (1975) vs. 4-7' (2019)

We can't know how many buses Perec missed in his tallies, but there's nothing to suggest he'd be more likely to miss a given line (it is heavily implied they all share the same stop). With that in mind, it stands out that the 86 is the only line where the 2019 intervals aren't 1/3 to 1/2 shorter.

On Sunday, Perec notes buses for 125m and tallies 10 63s and 8 96s (2019 numbers would have us expecting 6-10 63s and 12–18 96s). It seems like historically, the 63 ran more often on Sundays, but was reduced once the 70, 86, and 87 started running that day.