Calculate a Caltrans-style pavement Traffic Index from cumulative one-direction ESALs and a lane distribution factor, solve either input using unrounded TI, or make a simplified estimate from traffic inputs.

Enter cumulative one-direction ESALs before applying LDF. Do not apply the lane factor again to loads already assigned to the design lane.

Caltrans examples: 1.0 for one/two mixed-flow lanes; 0.8 outer and 0.2 inner lanes for three or more. Lane-use exceptions apply.

Related Calculators

Traffic Index Formula

The formula below gives raw TI. Caltrans HDM 613.2(5) specifies rounding TI upward to the next 0.5. Apply LDF once to cumulative one-direction loading; if ESALs are already assigned to the design lane, use LDF = 1. The traffic-input task estimates two-way ESALs as AADT ร— truck share ร— 365 ร— G ร— ESALs/truck, then applies D and LDF. Here G = [(1 + g)^years โˆ’ 1]/g, or years at g = 0, using first-year traffic and whole-year geometric growth. This simplified forecast is not the full axle-class or mechanisticโ€“empirical pavement design procedure.

TI = 9.0 ร— (ESAL ร— LDF / 10โถ) ^ .119

Variables:

  • TI is the Traffic Index
  • ESAL is the number of cumulative 18-kip equivalent single axle loads over the design life of the pavement structure 
  • LDF is the lane-specific distribution factor. Current Caltrans Table 613.3B uses 1.0 for one or two mixed-flow lanes and, for three or more, typically 0.8 for outer and 0.2 for inner lanes, with lane-use exceptions. These are not universal factors. 

How to Calculate Traffic Index?

The following steps outline how to calculate the Traffic Index.

  1. First, determine the number of cumulative 18-kip equivalent single axle loads over the design life of the pavement structure. 
  2. Next, determine the lane distribution factor. 
  3. Next, gather the formula from above: TI = 9.0*(ESAL * LDF / 10^6) ^ .119.
  4. Finally, calculate the Traffic Index.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

number of cumulative 18-kip equivalent single axle loads over the design life of the pavement structure = 35,000,000

lane distribution factor = .75

TI = 9.0 ร— (35,000,000 ร— 0.75 / 10โถ)^0.119 โ‰ˆ 13.2776; rounded upward to 0.5, TI = 13.5. The 0.75 factor is an illustrative input, not a Caltrans lane preset.