Complex sentence: Rather than simulating a steady state of flow for a journey, they simulate transient "demand peaks" of congestion, which they model by using small "time slices" across the network throughout the working day or weekend.
Split into simpler sentences: Rather than simulating a steady state of flow for a journey, transient "demand peaks" of congestion are simulated. These are modeled by using small "time slices" across the network throughout the working day or weekend.