Trucking's troubles could preview an economic downturn

Data: Cass Information Systems; Chart: Lazaro Gamio/Axios

Truck shipments' year-over-year growth fell in December for the first time since 2016, data from the Cass Freight Index showed.

Why it matters: Truck shipments are thought to be a leading indicator as they're very sensitive to market fears. When things are good, suppliers load up, but in hard times the oversupply of trucks and weak demand from shippers can lead to quickly declining rates and a prolonged downturn for the industry, often coupled with more widespread economic malaise.