Small changes in speeds also seem to have an outsize psychological impact on impatient New Yorkers. On weekdays, when few people expect a trip to go quickly, speeds in east Midtown average about 6.3 m.p.h. in the daytime. On Saturdays, the average speed is about 8.5 m.p.h. — not an enormous difference, even though drivers report feeling more comfortable on weekends.
We expect traffic to be lighter and therefore faster on days when people are less rushed (therefore less stressed). It is unclear whether or not there is a third cause (less weekend responsibility) that confounds the causal conclusion offered by the Times.