Traffic

The traffic loads are distributed on four axle types: steering (with a single wheel), single, tandem and tridem (all with dual wheels). For each time period and each lateral position (if wander is considered) the critical stresses and strains are evaluated for each load level for each of the axle types.

The load distributions are derived from Weigh-In-Motion (WIM) stations at more than 100 locations in California (Truck Traffic Analysis using Weigh-In-Motion (WIM) Data in California). An analysis of the load distribution data (Grouping of WIM Sites Based on Axle Load Spectra) showed that the load distributions fall into eight groups. The decision tree used to determine the group is shown below and in "corrected_flowchart".

The averaged traffic inputs from all WIM sites in each group were used as the traffic characteristics of the regions covered by the group. For CalME, the number of axles per truck and the hourly axle load spectra are averaged for each group. Other traffic inputs, including the number of axles per lane per year and growth rate are calculated for each highway section from the Caltrans annual AADTT report

http://www.dot.ca.gov/hq/traffops/saferesr/trafdata/

which is stored in the table "CaltransAADTT". The present number of axles in the design lane is calculated from the year of estimation in the table, using a linear growth rate.