What It Means
Average Daily Traffic (ADT) is the estimated total number of vehicles, including passenger cars, trucks, buses, and motorcycles, crossing a bridge in a 24-hour period, averaged across the year. In the NBI it is captured as Item 29 (ADT) with a companion field for the year of the count (Item 30) and truck ADT percentage (Item 109). Traffic counts are typically collected by state DOTs using pneumatic road tubes, inductive loops, or permanent automatic traffic recorders, then extrapolated to annual averages using seasonal and day-of-week adjustment factors. ADT values on NBI bridges range from fewer than 50 on rural single-lane county bridges to over 300,000 on urban interstate crossings like the George Washington Bridge in New York or the Woodrow Wilson Bridge near Washington, D.C. ADT is critical to bridge prioritization for several reasons. First, it drives the Sufficiency Rating formula through the essentiality for public use component (15% weight). Second, it determines how many people are exposed to risk from a deficient bridge, a Poor-rated bridge carrying 100,000 vehicles per day represents vastly more exposure than an identical bridge carrying 200. Third, ADT is used by FHWA and state DOTs to calculate user cost savings from bridge investments, feeding benefit-cost analyses for IIJA and Highway Bridge Program grants. High-ADT structurally deficient bridges receive elevated priority under the $40 billion IIJA bridge program. Truck ADT, typically 5-15% of total ADT on interstates and lower on local roads, is particularly important for load rating and fatigue analysis.
Average Daily Traffic (ADT) is one of the bridge-engineering or FHWA-policy concepts that recurs across BridgeSafety. Below is how the concept connects to the National Bridge Inventory data behind every page on the site.
Within the BridgeSafety Condition Score, each primary component (deck, superstructure, substructure) contributes about a third of the rating, with an age penalty applied to bridges past their typical design life. The methodology page describes the scoring in full detail.
Frequently Asked Questions
What does "Average Daily Traffic (ADT)" mean?
The average number of vehicles crossing a bridge per day, recorded as NBI Item 29 and used to prioritize maintenance and funding.
Why does Average Daily Traffic (ADT) matter for bridge safety?
Average Daily Traffic (ADT) is the estimated total number of vehicles, including passenger cars, trucks, buses, and motorcycles, crossing a bridge in a 24-hour period, averaged across the year. In the NBI it is captured as Item 29 (ADT) with a companion field for the year of the count (Item 30) and ...