The IStructE has a guide for calculating carbon has a very rough estimate for calculating A4 based on mass, distance travelled and method of transport. It’s a useful high level approximation which can indicate how much the transport of material can affect the embodied carbon value. We have found that for mass timber this can be a higher proportion of total compared to steel/concrete which tend to be sourced more locally (obv depending quantities, site location etc.). Note, the values are UK specific but should be somewhat similar in N.A.
https://www.istructe.org/resources/guidance/how-to-calculate-embodied-carbon/