30th hour volume and design controls

Why the 30th hour volume is used as the design volume

If the hourly traffic volumes of a year (8760 hours) are arranged in descending order and plotted as a percentage of the AADT, the curve falls steeply at first and then flattens, forming a knee at about the 30th hour (the 30th highest hourly volume is about 12-18% of AADT on rural roads and 8-12% on urban roads).

 Hourly volume
 (% AADT) |\
          | \
          |  \_______ knee at ~30th hour
          |          ------------
          +---------------------- hours (descending)

• Designing for the highest hourly volume would make the road very wide and costly, yet that capacity would be used for only a few hours in a year.
• Designing for a much lower volume (e.g. the 100th hour) would cause congestion during many more hours.
• To the left of the knee, the volume rises steeply, so designing for an earlier hour greatly increases the cost while serving only a few extra hours. To the right of the knee, the curve is flat, so a later hour saves little. Hence the 30th highest hourly volume is a good economic balance: it is exceeded in only 29 hours of the year.

Other design controls (briefly)

1. Design speed: fixes the radius, sight distance, superelevation and gradients.
2. Topography: plain, rolling, hilly, mountainous.
3. Traffic volume and composition: number of lanes and capacity (PCU), proportion of trucks.
4. Design vehicle: its dimensions and turning path decide the lane width, radius and widening.
5. Human factors: reaction time, eye height, driver behaviour.
6. Environmental factors: noise, air pollution, aesthetics, land use.
7. Economic factors: available funds, benefit-cost.
8. Road class and safety: NH, feeder, district roads (NRS 2070).