and the impact on costs in other fuel- and energy-related areas, operating margins could be impacted. Whether fuel prices fluctuate or remain constant, operating results may be adversely affected if competitive pressures limit our ability to recover fuel surcharges. In periods of declining fuel prices, fuel surcharge percentages also decrease, which negatively impacts the total billed revenue per hundredweight measure and, consequently, revenues. The revenue decline may be disproportionate to the change in our fuel costs. The segment’s operating results will continue to be impacted by further changes in fuel prices and related fuel surcharges.
Asset-Based Operating Income
The Asset-Based segment generated operating income of $72.8 million and $126.2 million for the three and six months ended June 30, 2024, respectively, compared to $43.3 million and $90.8 million, respectively, for the same periods of 2023. The Asset-Based segment’s operating ratio improved by 4.2 percentage points and 2.7 percentage points, respectively, for the three and six months ended June 30, 2024, compared to the same prior-year periods, primarily reflecting decreased operating expenses and cost control efforts to reduce utilization of outside resources and to optimize internal resources and total cost per mile to serve the business, as further discussed in the following paragraphs.
Asset-Based Operating Expenses
Labor costs, which are reported in operating expenses as salaries, wages, and benefits, amounted to 49.5% and 50.4% of Asset-Based segment revenues for the three- and six-month periods ended June 30, 2024, respectively, compared to 47.7% and 47.9%, respectively, for the same periods of 2023. The increase in salaries, wages, and benefits as a percentage of revenue for the three and six months ended June 30, 2024, compared to the same prior-year periods, was more than offset by lower utilization of purchased transportation as discussed later in this section. Salaries, wages, and benefits increased $8.1 million and $17.5 million for the three and six months ended June 30, 2024, respectively, compared to the same periods of 2023, primarily due to a 13% increase in union wages related to the wage and mileage rate increases effective July 1, 2023 under the 2023 ABF NMFA, as previously discussed in the Assed-Based Revenues section, partially offset by improved productivity as measured by shipments per DSY hour and a decrease in headcount to align with lower shipment levels. The increases in labor costs also reflect year-over-year increases in contractual benefit contribution rates under the labor agreements with the IBT. The average health, welfare, and pension benefit contribution rate increased approximately 3.7% effective primarily on August 1, 2023.
The Asset-Based segment manages costs with shipment levels; however, a number of factors impact DSY productivity, including the effect of freight profile and mix changes, utilization of local delivery agents, and efficiency of personnel. Shipments per DSY hour improved 7.4% and 5.0% for the three and six months ended June 30, 2024, respectively, compared to the same periods of 2023, primarily due to a decrease in dynamic LTL-price-quoted shipments, which typically require a higher number of stops, during the three and six months ended June 30, 2024. The improvement in shipments per DSY hour is also partially attributable to the separation of city operations from certain distribution centers in late 2023 and during the first half of 2024 and training and development at certain key locations, as the process compliance team continues to implement operational best practices throughout the Asset-Based network. For the three and six months ended June 30, 2024, the year-over-year decrease in pounds per mile of 4.2% and 4.4%, respectively, reflects lower weight per shipment, partially offset by an increase in the average length of haul.
Fuel, supplies, and expenses as a percentage of revenue decreased 1.0 percentage point and 1.3 percentage points during the three and six months ended June 30, 2024, respectively, compared to the same periods of 2023. Fuel expense decreased during the 2024 periods, as the Asset-Based segment’s average fuel price per gallon (excluding taxes) decreased 1.9% and 6.6%, respectively, for the three and six months ended June 30, 2024, compared to the same period of 2023. Lower city tractor age contributed to the decrease in costs to repair and maintain revenue equipment units during the first half of 2024, compared to the first half of 2023.
Insurance as a percentage of revenue increased 0.6 percentage point and 0.5 percentage point during the three and six months ended June 30, 2024, respectively compared to the same periods of 2023, primarily due to an increase in the severity of third-party casualty claims, including an increase in the number of large claims in recent years.
Rents and purchased transportation as a percentage of revenue decreased 4.2 percentage points and 3.8 percentage points for the three and six months ended June 30, 2024, respectively, compared to the same periods of 2023, primarily due to focused reduction in the utilization of local delivery agents and linehaul purchased transportation. Rail miles decreased