Illinois Bridge Law: Axle Limits and the 45'6" Kingpin Rule

Illinois adopted the federal limits statewide — 20,000 single, 34,000 tandem, 80,000 gross, federal bridge formula — and its kingpin rule is one of the most generous in the country at 45 feet 6 inches to the center of the rear axle on Class I and II highways. For a 53-foot van it is rarely the binding constraint.

By Avery Schwartz · Former CDL-A driver, 3 years OTR · Updated 2026-09-15

The numbers at a glance

Limit	IL	Where it comes from
Gross weight	80,000 lb	625 ILCS 5/15-111
Single axle	20,000 lb	15-111
Tandem axle	34,000 lb (no axle over 20,000)	15-111; 36-ft rule for two tandems
KPRA / wheelbase	45 ft 6 in, kingpin to center of rear axle	625 ILCS 5/15-107; semitrailers over 48 ft on Class I/II; 42'6" on Class III per J.J. Keller
Bridge formula applies?	Yes — statewide	15-111 contains W = 500(LN/(N−1) + 12N + 36) and tables

Legal limits change and enforcement varies by road. Confirm anything you're betting a ticket on with IDOT / Illinois State Police.

How Illinois applies the bridge law

Illinois used to be a patchwork of designated and non-designated routes with different gross limits, but the modern statute is clean: 625 ILCS 5/15-111 sets 20,000 lb on a single axle, 34,000 lb on a tandem with no axle in it over 20,000, and 80,000 lb gross for combinations of five or more axles, all governed by the federal bridge formula and its weight tables. The 36-foot rule is written in: two consecutive tandems may each carry 34,000 if the first and last axles are at least 36 feet apart.

The kingpin rule lives in the length section, 625 ILCS 5/15-107. For a semitrailer longer than 48 feet in combination with a tractor, the distance from the kingpin to the center of the rear axle may not exceed 45 feet 6 inches on Class I and Class II highways. J.J. Keller's KPRA table lists 42 feet 6 inches for Class III highways; verify that figure with IDOT if you leave the designated system. Either way, a 53-foot van with tandems fully back typically measures in the low 40s to center, so Illinois's rule is rarely what limits you — weight is.

Enforcement is active. Illinois State Police runs fixed scales on the major Interstates and its truck-enforcement officers are known for portable-scale details on state routes. Local police in Illinois also have overweight authority on their own roads, which is unusual and worth remembering when you leave the Interstate for a delivery.

How to slide your tandems for Illinois

Illinois rarely binds on kingpin, so the slide calculator is the whole answer: find the heavy group, move the tandems toward it, confirm the drives-to-trailer span is 36 feet or more if both tandems are near 34,000. If you are continuing east into Indiana (43 ft to rearmost axle) or Ohio, check those pages before you settle on a hole.

Worked example. You scale at the first CAT after the shipper: steer 11,800, drives 35,300, trailer tandems 31,900, gross 79,000. Gross is legal; the drives are 1,300 over. Sliding the tandems forward (toward the cab) moves weight from the drives to the trailer axles. On a trailer with 4-inch hole spacing, a hole is worth roughly 250 lb, so 1,300 lb needs six holes: drives about 33,800, tandems about 33,400. Both groups are under 34,000 with a few hundred pounds of margin, and the gross hasn't changed. Now check the geometry for Illinois using the row in the table above — and if you'll cross into a stricter state, set for that one instead. The Tandem Slide Calculator does the hole math for your trailer's actual rail spacing and tells you which direction to move.

Common tickets in Illinois

Based on the statutes above and what drivers commonly report; citation practice varies by officer and county, so treat this as a watch list, not a fine schedule.

Tandem or gross overweight — Illinois overweight fines are calculated per pound over and are among the steeper schedules in the Midwest; verify the current schedule with ISP.
Local-road overweight — municipal and township roads may be posted below state limits and local officers enforce them.
Non-designated route with a 53-foot trailer — the 45'6" rule and 53-ft length apply on Class I/II highways; other roads have their own limits.

Illinois bridge law FAQ

What is the Illinois kingpin-to-rear-axle limit?

45 feet 6 inches from the kingpin to the center of the rear axle, for semitrailers longer than 48 feet on Class I and Class II highways (625 ILCS 5/15-107). A shorter limit is reported for Class III highways; verify with IDOT.

Is Illinois 80,000 lb on all roads now?

Under 15-111, the federal limits and bridge formula apply on the highways of the state, with 80,000 lb gross for five or more axles. Locally posted roads can be lower.

Does the 36-foot rule apply in Illinois?

Yes. Two consecutive sets of tandem axles may carry 34,000 lb each only if the overall distance between the first and last axles of the two tandems is 36 feet or more.

Can a city police officer weigh my truck in Illinois?

Illinois law gives local police overweight enforcement authority on roads under their jurisdiction, and it is commonly used. Treat posted local limits as enforced.

Sources

This page is a driver's summary, not legal advice. Statutes are quoted or paraphrased from the sources listed; where we couldn't verify a figure from an official source we've said so.