I have a large set of traffic data (55000+ rows and 15 columns). Each row
contains the data of one vehicle passing through a survey location.

The columns include the time at which the vehicle passed the survey
location, it's velocity, a property called headway etc...

I would like to work out the average velocity and headway of the vehicles
passing through the survey location for each hour (remembering that the
number of vehicles and hence rows is different for every hour).

Any ideas?