The influence of traffic as an input parameter for mechanistic-empirical (M-E) pavement analysis on the percentage of fatigue cracks on slabs or the required slab thickness for jointed concrete pavements (JPCP) was investigated. Site-specific traffic data from Illinois weigh stations exhibiting different axle load and vehicle class distributions were compared against the default traffic inputs used in the M-E pavement design program. The load spectra for several sites were adjusted by changing the annual average daily truck traffic (AADTT) volumes to produce the same equivalent single axle loads (ESALs) for a typical interstate highway design in Illinois. The influence on the required JPCP thickness varied less than 2.0 inch (50 mm) for combinations of all traffic distributions and climate regions given the same level of ESALs. For a given climate station with a fixed ESAL level, there was no significance in cracking level for the load spectra data gathered at different stations across Illinois. Only in extreme overload traffic cases, where a significant percentage of the single axle weights were 50 percent greater than the legal limit, did the distribution of traffic significantly influence the slab cracking performance for a fixed number of ESALs and climate. For JPCP design, the difference between using an ESAL-based design and load spectra design for fatigue cracking prediction was determined to be negligible when using the M-E pavement design program with existing Illinois traffic inputs and a realistic magnitude of overloads.