Big trucking companies, scrambling along with their customers to keep up with fast-growing U.S. economic demand, are plowing millions of dollars into technology to build up services as digital freight middlemen.

The growing investment in online freight brokerage is aimed at building on operations many truckers have used to supplement their core transportation businesses, even when that means turning shipments over to other truckers or independent drivers.

The expansion is being driven by customer demand and by the need to compete with a growing lineup of upstart companies that are using new technology to grab a slice of a U.S. freight market that’s growing at a fevered pace.

Like the Uber Freight unit at Uber Technologies Inc. and load-matching startups like Seattle-based Convoy, the trucking companies’ platforms aim to automate tasks like phone calls and booking shipments. They also harvest information that trucking companies say can help find space for customers grappling with one of the tightest freight markets in years.

“This is a new way for us to think about getting efficiency into and waste out of a very large system that is under a lot of pressure today,” said John Roberts, chief executive of J.B. Hunt Transport Services Inc., one of the largest carriers in North America. The Lowell, Ark.-based company’s online marketplace generated about $137m in the second quarter, up from $96m the previous quarter, and accounted for about 40 percent of its overall brokerage revenue.