TRAFFIC DESIGN VOLUMES
How to select a design-hourly volume? Problem: Variability in traffic volumes by time of day, day of week, time of year, and type of roadway.
Examples of hourly variation
In t r a c it y R o u t e
1 0 .0 8 .0 6 .0 4 .0
Percent of Daily Traffic
2 .0 1 2 :0 0 A M 6 :0 0 A M 1 2 :0 0 P M 6 :0 0 P M 1 2 :0 0 A M
In t e r c it y R o u t e
1 0 .0 8 .0 6 .0 4 .0 2 .0 1 2 :0 0 A M 6 :0 0 A M 1 2 :0 0 P M 6 :0 0 P M 1 2 :0 0 A M
T im e o f D a y
Monthly Traffic Volume Variations
150 140
Monthly Average Daily Traffic as Percent of AADT
130 120 110
AADT
100 90 80 70 60 Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Month
Solution:
Work with percentages of the annual average
daily traffic (AADT)
AADT is in units of vehicles per day (for both
travel directions) and computed as the total yearly traffic volume divided by the number of days in the year.
Convert AADT into Design Hourly Volume
(DHV)
How to pick a design hourly volume
Hourly volume as a proportion of AADT 0.15
0.14
Rank Highest 100 Hourly Volumes Over a OneYear Period for a Typical Roadway Design practice in the United States uses a design hourly volume (DHV) that is between the 10th and 50th highest volume hour of the year. Most common hourly volume used for roadway design is the 30th highest volume hourly of the year.
0.13
0.12 K30 = 0.12 0.11
0.10 0 20 40 60 80 100 Number of hours (annually) with specified or greater volumes
K-factor
In practice, the K-factor is used to convert annual average daily traffic (AADT) to the hourly volume:
DHV K = AADT
DHV: design hourly volume (typically, the 30th highest annual hourly volume) AADT: the roadway's annual average daily traffic K: a factor to convert annual average daily traffic to annual hourly volume
DDHV
For freeways and multilane highways, interest is in directional traffic flows. Need a factor to reflect the proportion of peak-hour traffic volume traveling in the peak direction. Get the directional design-hour volume (DDHV) by application of:
DDHV = D * K * AADT
DDHV: directional design-hour volume D: directional distribution factor to reflect the proportion of peak-hour traffic volume traveling in the peak direction
Example
A 4-lane freeway (2 lanes in each direction) segment consists of: passenger cars only driving population of regular users peak-hour directional distribution of 0.7 peak hour factor of 0.8 measured free-flow speed of 70mph Assuming Fig. 6.7 applies, if the AADT is 30000 veh/day, determine the level of service for the 10th, 50th, and 100th highest annual hourly volumes.
Determine K
Level of Service