What is California's 40-Foot KPRA Rule?

The 40-foot kingpin-to-rear-axle regulation involves the distance from the center of the rear tandem axle to the kingpin. The distance between these components cannot be any longer than 40 feet. This measurement is taken from the center of the tandem axle to the kingpin’s center; in California, any length further than this is illegal.

The distance between the axle and kingpin directly impacts the weight distribution across the truck’s wheels and axles. This influences the truck’s stability, directly affecting road safety and the driver’s ability to comply with legal weight constraints.

Adjusting the Rear Tandem Axle

To change the distance between the tandem axle and kingpin, just slide the rear tandem all the way forward.
You may also be able to move the axle to a specific position (e.g., the fifth or sixth hole).

KPRA Rules Improve Safety and Reduce Infrastructure Damage

The goal of the KPRA measurement rule is to balance public safety and vehicle performance. Ultimately, protecting truck operators, the public, and infrastructure.

In addition, since proper axle placement distributes the weight more evenly, it helps prevent road damage.

Proper Axle Placement Improves Vehicle Stability

If a driver improperly places a rear axle, the trailer's center of gravity changes. This increases the risk of an accident.

California’s KPRA Guidelines:

• Semitrailer length has no limit.
• A trailer with two or more axles has a 40-foot maximum.
• Single-axle trailers have a 38-foot maximum.
• Maximum overall length is 65 feet.

California’s Vehicle Weight and Axle-Specific Rules

Statewide, California’s gross vehicle weight allowance is 80,000 pounds. In addition, there are axle-specific rules for truck weight.

Axle-specific rules:

• Steering axles — 620 pounds for each inch of tire width.
• Tandem axles — a maximum of 34,000 pounds (with appropriate tire ratings).

KPRA’s axle placement regulations also help improve braking efficiency.

Length braking distances for good stability at 60 mph:

• KPRA length of 35 feet — start to brake at 220 feet (excellent).
• KPRA length of 40 feet — start to brake at 250 feet (good).
• KPRA length of 45 feet — start to brake at 290 feet (moderate).

According to the National Highway Traffic Safety Administration (NHTSA), there has been a 15% reduction in jackknife accidents. They attribute this reduction to the shorter axle-to-kingpin measurement rule.

The Importance of Following KPRA

When operators abide by the KPRA rule, their turns are more stable, which helps prevent tail swings. In addition, proper weight distribution prevents 80% of the bridge strikes that occur in high-wind zones. Furthermore, according to inspection logs from California’s Highway Patrol, 22% of combination vehicle crashes that result in fatalities involve an excess tail swing.

Calculators Can Assist With Route Planning

Drivers can use an online Federal Bridge Formula Calculator to determine the maximum amount of weight each set of their truck's axles can legally carry on the Interstates and roadways.

California Has Route Restrictions for Commercial Vehicles

More than 20 segments have bans that affect commercial vehicles that are carrying hazardous materials or an oversized load. Therefore, to successfully navigate California’s roadways, drivers must be aware of the state's route-specific rules. Drivers can use online tools like Caltrans QuickMap to stay up-to-date about rules as well as changes on the Interstate and roadways.

Caltrans QuickMap

Caltrans QuickMap provides real-time alerts for road closures and weigh stations. Additionally, since this tool integrates with Electronic Logging Device (ELD) systems, it can flag restricted routes even before a driver is dispatched.

Some key features of QuickMap are:

• Mobile alerts about temporary limit changes.
• Databases for low-clearance bridges (updated hourly).

Some of California’s highway restrictions:

• On I-80, tandem vehicle combinations are subject to weight checks.
• Ortega Highway (State Route 74) has strict trailer length limits.
• In Antelope Valley, trucks transporting radioactive materials have to follow designated paths.
• From 3 to 5 am, the Caldecott Tunnel bans trucks from transporting explosives.

Fines and penalties are costly, and repeat offenders can cause insurance rates to rise. Therefore, it's crucial that truck drivers know the state and federal regulations so they can follow them. You can take advantage of the free online tools, apps, and blogs that are designed to help you stay on target.