section 2.0 Policy Implications; section 2.4 Policies to Encourage Tridems

Navigation: Page 1, Cover Page | Page 2.. Phase 1.1, Background. | 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights | Page 4, Section 1.2 [b] Tire Characteristics | Page 5, Section 1.2 (c) Suspension Systems | Page 6, Section 1.2; (d) Axle Spacing | Page 7, Section 1.2; (e) Liftable Axles | Page 8, Section 1.2; (f) Tridem Axles | Page 9, Section 2.1 Axle Weight Limits | Page 10, Section 2.2 Bridge Formula | Page 11, Section 2.3 - 80,000 Pound GVW Cap | Page 11, Section 2.4 Policies to Encourage Tridems | Page 11, section 2.5 Weight Limits Per Unit of Tire Width | Page 12, section 2.6 Turner Trucks | Page 13, section 2.7-New Approach; TRB Truck Weight Study | Page 14, section Section 3.0; Knowledge Gaps and Research Needs | Page 15, section 4.0 References for Pavements Working Paper

Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

2.0 Policy Implications

2.4 Policies to Encourage Tridems

When viewed using the AASHTO load-equivalence factors, combinations with tridem axles generally have much lower pavement costs per ton of freight carried than conventional five axle combinations. As shown in Exhibit 2, a six-axle tractor-semitrailer with a rear tridem carrying 88,000 pounds produces 1.88 ESALs on flexible pavements and 3.57 ESALs on rigid pavements. The corresponding ESAL values for a conventional five axle tractor-semitrailer carrying 80,000 pounds are 2.37 (flexible) and 4.07 (rigid). Assuming tare weights of 28,000 and 29,500 pounds for the five- and six-axle combinations, ESALs per ton of payload for the trucks shown in Exhibit 2 are as follows:

	ESALs per million pounds of payload
	Flexible pavement	Rigid pavement
5-axle tractor-semi	46	78
6-axle tractor-semi	32	61

However, much of the pavement benefits shown in the above table disappear if load equivalence factors consistent with Hudson and Buttler's findings (discussed above in Section 1.2.f) are assumed. Specifically, for flexible pavements, the reduction in ESALs per million pounds of payload would drop from 14 to roughly 4. For rigid pavements, the reduction in ESALs per million tons of payload would drop from 17 to roughly 11. Thus, if Hudson and Buttler's conclusions are correct, it appears that there are still pavement cost savings to be realized by promoting a shift to tridems. However, these savings are far less than would be anticipated using the AASHTO load equivalence factors.

[continued on next page, Page 11, section 2.5 Weight Limits Per Unit of Tire Width ]

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