ABSTRACT Polymethyl methacrylate (PMMA) modified with carbon nanotubes (CNTs) shows promise as a piezoresistive material for road pavements. The micromechanics model holds significance in designing and optimising the piezoresistive performance of CNTs-modified PMMA. In this paper, the excluded volume is introduced to characterise the effect of mineral fillers, as the main components in PMMA mastic, within the micromechanics model. The investigation integrates Scanning Electron Microscopy (SEM), Monte Carlo numerical simulation, and electrical conductivity tests. Subsequently, a parameter analysis is conducted to optimise the piezoresistive sensitivity. SEM results reveal two representative interphase morphologies and identify the excluded volume of mineral fillers. It is estimated as 19.4% of the solid volume of mineral fillers through the numerical simulation. Moreover, the modelled electrical conductivity of PMMA mastic using the improved model precisely aligns with experimental results. These could validate and quantify the excluded volume of mineral fillers and then improve the micromechanics model for piezoresistivity. The parameter analysis demonstrates that mineral fillers markedly enhance the piezoresistive sensitivity of PMMA mastic. It is suggested to prepare PMMA mastic with a lower Poisson’s ratio and volume fraction of CNTs. This improved micromechanics model could guide the design and optimisation of piezoresistive polymers in intelligent road systems.