Thanks for your question. It depends on the LCI source selected. Early LCI Sources we had trouble finding references and ended up back-calculating utilisation etc from aggregated transport impact per mode at national level. Later data sources there has been more information available. For example, the eTool Default, UK V14 data source uses the RICS Guide (which draws from ILCD). There has been some scrutiny of the transport distances during a number of external reviews and thus far it’s stood up to the scrutiny.
Unfortunately the IMPACT data is a bit of a black box, however they adhere to the BRE product category rule for construction products which may have more information: https://www.greenbooklive.com/filelibrary/EN_15804/PN514-BRE-EN-15804-PCR.pdf I had a brief look but couldn’t find anything explicit in there (however it was brief so I possibly missed something).