23 CFR §1340.6

1. (a)Daylight hours. All daylight hours between 7 a.m. and 6 p.m. for all days of the week shall be eligible for inclusion in the sample.
2. (b)Random assignment. Except as provided in paragraph (c) of this section, the day-of the week and time-of-the-day shall be randomly assigned to observation sites.
3. (c)Grouping of observation sites in close geographic proximity. Observations sites in close geographic proximity may be grouped to reduce data collection burdens if:
   1. (1)The first assignment of an observation site within the group is randomly selected; and
   2. (2)The assignment of other observations sites within the group is made in a manner that promotes administrative efficiency and timely completion of the survey.