Impact of Weight Limits on Wisconsin's Highways: Insights from the DOT

1. Highway Weight Limits Wisconsin Department of Transportation Response to Questions from the Special Committee on Weight Limits November 13, 2006

2. The Effect of ESALs • A method of expressing the effect of loads on pavement relative to an 18,000 pound single axle • A six axle truck at 120,000 GVW has about the same impact on a pavement as a five axle 98,000 GVW truck • Either of those loads cause about 400% more pavement damage than a 90,000 GVW, six axle truck

3. Harder Pavement Aggregates WisDOT specifies high quality aggregates suitable for the pavement design Designing pavements for heavier loads can be accomplished in a number of ways Limiting aggregates to a single type or source is cost prohibitive

4. Effect of Wider Tires • Tire loads affect localized areas • Narrow width tires concentrate weight on a small area • Wider tires distribute the weight over a larger area and cause less stress on a single spot • Axle loads are relevant regardless of the spot stresses • Total loads may cause damage or failure even if the local point stresses are not large • Refer to Washington DOT material for their description

5. Effect of Changing from 73,000 to 80,000 pound limit for cement trucks • No specific information • Assuming truck configurations, travel speed, tire pressure, and vehicle suspension were unchanged, the roughly 10% increase in load would increase pavement wear by perhaps 80%, possibly offset or mitigated by trip reduction • Rough relationship reflects the geometric relationship between increased loads and impacts on infrastructure

6. Factors influencing life cycle of roads and bridges • Loads • Capacity and uses • Environment

7. Comparing Standards • Wisconsin • Majority of bridges designed for 36 tons or less (HS15 or HS20) • Michigan • Typically design for HS25 truck (90,000 pound GVW) – although max legal weight is 164,000 • Law requires very specific axle spacing and weight configurations • Bridge postings are subject to considering 31 various legal loads on each bridge (versus just 3 for most states) – result is more posted bridges

8. Comparing Systems • MI system is showing the effects of heavy loads • Deficient bridges much more common • MI 28% of bridges deficient • WI 16% of bridges deficient • Reflects accelerated wear and also generates need for more intensive maintenance and rehabilitation at greater cost

9. Conditions of Local Roads • Bridge information is available for local roads • Other local road information for pavements is just being accumulated and detailed information about roadway structure is not readily available • Many local roads are not designed or constructed for heavy loads

10. Costs of Illegal Overweight Vehicles to the State • Unable to provide a reliable estimate due to inadequate knowledge of: • Number of illegal loads • Miles driven by illegal loads • Roads impacted by these loads