General Approach for Traffic Input

Similar to other M-E design procedures, traffic inputs are defined by specifying the traffic volume and the load spectrum for trucks in CalME. Passenger cars are ignored in pavement structure designs because they are much lighter and as a result cause much less damage to pavements structures compared to trucks. 

Traffic volume in CalME is specified by entering the design traffic index (TI), which is correlated to the number of Equivalent Standard Axle Load (ESAL) through the following equation (adapted from Caltrans Highway Design Manual):

The conversion of truck traffic to ESAL count follows the standard equations. CalME uses a power of 4.2 following Caltrans practice.

Load spectrum refers to the distribution of truck traffic over time and load level. A load spectrum provides all the details needed for spreading the traffic volume into traffic loads needed for calculating pavement responses and in turn predicting the traffic induced pavement damages. The combination of traffic volume and load spectrum fully specifies the mixture of traffic loads to be applied on a pavement structure for any given time period. In order to calculate pavement response, one needs to know the tire layout, tire pressure, load direction, and load amplitude. In order to predict damage, one needs to also know the number of applications of each traffic load.

It takes a large set of numbers to fully define a load spectrum, even after accounting for the cyclic patterns of tuck traffic. To make it easy, CalME has a set of pre-defined load spectra for designers to choose from. These spectra were developed after reviewing data from hundreds of weight-in-motion (WIM) stations installed in California highway (Truck Traffic Analysis using Weigh-In-Motion (WIM) Data in California).