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Chapter VIII

Traffic Operations

Western Uniformity Scenario Analysis graphic depicting western states and illustration of longer combination vehicle


Larger and heavier trucks affect traffic basically in two ways. Because of their size, weight and operating characteristics such trucks will reduce the "quality" of traffic flow and, in most cases, increase the number and severity of crashes. To describe the "quality" of highway traffic flow, transportation engineers developed the concept of Level of Service (LOS), with ratings from LOS A to LOS F, where LOS A reflects uninterrupted flow, that is, where the movements of any one vehicle does not effectively influence the travel of other vehicles. LOS E reflects that the highway is operating at capacity, while LOS F reflects unstable flow where there is "stop and go" operation. Because of their size, acceleration and braking characteristics larger trucks negatively affect the roadway's LOS.

Secondly, these truck effects on the traffic stream not only impinge on flow quality, but they also affect safety in several ways. In addition to the obvious impact on crash severity due to truck weight, research has shown that speed differential among the vehicles in the traffic stream increases the probability of crashes. Because of trucks' poorer acceleration capability (as compared to passenger cars and other smaller vehicles with lower weight-to-horsepower ratios) the effects of any posted differential speed limits are magnified. Generally, traffic operations degrade as the proportion of trucks in the traffic stream increases, and as the acceleration and stopping distance differentials between trucks and other vehicles increase.

This chapter presents qualitative assessments of the traffic operations impacts of the Western Uniformity Scenario trucks in the 13 analyzed States. Although traffic originating or terminating outside the region that shifts from a single-trailer combination vehicle to a "scenario LCV" may change its travel route outside the region to access the Western Uniformity Scenario network, these volume shifts will be negligible. This is because the Scenario network is comprehensive and connectivity to the outside-the-region network is pervasive. In addition to these minor route changes, traffic that originates or terminates outside the region and diverts from a 53-foot single-trailer vehicle in the Basecase Scenario to an LCV in the Western Uniformity Scenario may generate an increase in the number of truck trips outside the region. Under the Western Uniformity Scenario, no LCV configuration allows a 53-foot trailer and the analysis assumes that trailers conforming to the Uniformity length regulations are paired up at the region's border and not reloaded from non-conforming trailers. Such changes in truck travel outside the region affect only a very small amount of the overall truck travel.

Vehicle Characteristics and Their Affect on Traffic Flow and Safety

Acceleration and Speed Maintenance

Acceleration performance determines a truck's basic ability to blend well with other vehicles in traffic. Poor acceleration or speed maintenance is a concern as it results in large speed differentials between vehicles in traffic, and crash risks increase significantly with increasing speed differentials. The Comprehensive Trucks Size and Weight Study (CTS&W) Volume III showed that crash involvement might be 15 - 16 times more likely at a speed differential of 20 miles-per-hour than when there is no difference in speeds. Also poor acceleration performance increases vehicle interaction and subsequent delay, thereby degrading the LOS.

Engine manufacturers have responded to the needs of heavier trucks by building engines with up to 600 horsepower. These engines are sufficient to maintain a minimum speed of 20 mph for a 130,000 pound truck on a 6 percent grade. This provides sufficient power to allow these vehicles to operate in conformity with Federal policy standards for the Interstate System. For example, Federal policy states that highways with design speeds of 70 mph may not have grades exceeding 3 percent. However, gradients may be up to 2 percent steeper when in rugged terrain. Table VIII-1 shows the engine horsepower necessary to yield selected weight-to-horsepower ratios. This table provides a point of reference as to the horsepower required for vehicles operating at increased weights that maintains the weight-to-horsepower ratio of the lower weight vehicle. For examqle, to maintain the ratio of 250 an 80,000-pound 5-axle tractor-semitrailer combination needs a 320 horsepower engine, but an LCV loaded to the Western Uniformity Scenario maximum weight of 129,000 pounds would require a 516 horsepower engine. Although the 600 HP engine permits LCVs to operate in a similar fashion to most single trailer trucks, it is not sufficient for a fully loaded 18-wheeler with a 450+ HP engine, which is not uncommon among such trucks.

Table VIII-1
Horsepower Requirements
Select Weight-to-Horsepower Ratios and Gross Vehicle Weights
Weight/Horsepower Ratio (pounds) Horsepower Required for Weight-to-Horsepower Ratio in Right Column
Typical 3S2* Tare Weight 30,000 lbs Typical 3S2* Partial Load 60,000 lbs Maximum 3S2* Load 80,000 lbs Triples Uniformity Weight 110,000 lbs Typical Uniformity 8-axle LCV 120,000 lbs Maximum Uniformity LCV 129,000 lbs
150 200 400 533 733 800 860
200 150 300 400 550 600 645
250 120 240 320 440 480 516

*3S2 is a 5-axle tractor semitrailer with 3-axles on the tractor and 2-axles on the semitrailer.

Size and Acceleration Impacts on Congestion

Trucks are larger and, more importantly, accelerate more slowly than passenger cars, and thus have greater impacts on traffic flow than passenger cars. In the CTS&W Study Volume III, the impact on traffic congestion was assessed in terms of changes in passenger car equivalents (PCE). A PCE represents the number of passenger cars that would use the same amount of highway capacity as the vehicle being considered under the prevailing roadway and traffic conditions.

A significant variable for acceleration and speed maintenance is the grade or steepness of the road. The mountainous western States in this study contain a preponderance of steeply graded rural Interstate. CTS&W Study Volume III reports that on level terrain and in uncongested conditions conventional trucks may be equivalent to about two passenger cars, but on hilly or mountainous terrain and in congested traffic, their effect on traffic flow is much greater and may be equivalent to 15 or more passenger cars. Table VIII-2 shows PCE values for trucks operating in rural and urban areas under different conditions. The Rural portion of Table VIII-2 indicates the marked effect that percent and length of grade have on truck climbing ability if the truck has a high weight-to-horsepower ratio. Likewise, the urban portion of the table indicates that congested traffic conditions increase PCEs relative to uncongested conditions.

Table VIII-2
Truck Passenger Car Equivalents
Truck Length (feet)
        40 80 120
Interstate 0 % 0.50 mi 150 2.2 2.6 3.0
200 2.5 3.3 3.6      
250 3.1 3.4 4.0      
3 % 0.75 mi 150 9.0 9.6 10.5  
200 11.3 11.8 12.4      
250 13.2 14.1 14.7      
0 % 0.50 mi 150 1.5 1.7 Not Simulated
200 1.7 1.8 Not Simulated      
250 2.4 2.7 Not Simulated      
4 % 0.75 mi 150 5.0 5.4 Not Simulated  
200 8.2 8.9 Not Simulated      
250 13.8 15.1 Not Simulated      
Truck Length (feet)
      40 80 120
  Traffic Flow        
Other Freeways,
Uncongested 150 2.2 2.6 3.0
200 2.5 3.3 3.6    
250 3.1 3.4 4.0    
Congested 150 9.0 9.6 10.5  
200 11.3 11.8 12.4    
250 13.2 14.1 14.7    
Other Principal Arterial Average Conditions 150 1.9 2.2 2.4
200 2.0 2.2 2.6    
250 2.4 2.7 3.2    


If a tractor with an engine of insufficient capacity is used to provide motive power for a longer and heavier truck operating under size and weight limits of the Western Uniformity Scenario, the vehicle could take more time to accelerate into the traffic stream from a complete stop at a stop sign or a signalized intersection than the alternative Status Quo vehicle. The Western Uniformity Scenario increases off-Interstate weight limits for RMDs in nine of the thirteen States studied. In addition, Scenario RMD length limits increase in two of the nine States with increased RMD weights.37

Off-Interstate intersections pose potential challenges for increased RMD weight and length. Heavier and longer trucks turning onto an intersecting roadway, or crossing an intersection from a stopped position, will take longer to get up to traffic-flow speed or to clear the intersection than a lighter, shorter vehicle unless the vehicle horsepower is increased proportionately to maintain acceleration rates. Any additional time spent accelerating to flow speed after a turn or crossing an intersection would increase the risk of collision for through vehicles approaching intersections where sight distances are limited by physical features such as curves, hills, signage and foliage. LCVs crossing intersections from a stopped position could increase the distance required for the driver of a vehicle in cross traffic to see the truck and bring the vehicle to a stop to avoid a collision by up to ten percent.

The Western Uniformity Scenario mitigates or completely eliminates traffic impacts, relative to vehicles in the current fleet, related to the braking capability of trucks. Scenario weight limits for individual axles and axle groups are restricted to Federal limits, the same as Status Quo limits. For freight shifts from one configuration to another - for example from a 5-axle tractor-semitrailer at 80,000 pounds to a 9-axle TPD at 129,000 pounds - the gross vehicle weight per braking axle will generally decrease, thereby reducing braking demand on individual axle groups.

Passing on Two-Lane Roads

Cars passing RMDs on two-lane roads need up to 8 percent longer passing sight distances compared to passing tractor-semitrailer combinations. The Western Uniformity Scenario significantly expands the off-Interstate RMD's network in only four of the thirteen states - Oklahoma, Kansas, Nebraska and Colorado.38 No-passing zones for the expanded off-Interstate portions of the RMD network in these four States would need to be reengineered to maintain the current level of safety of passing single-trailer combinations for passing RMDs.

For their part, longer and heavier trucks would also require longer passing sight distances to safely pass cars on two-lane roads. Of the remaining nine states not adding significant RMD network mileage, five would increase RMD weight limits. These five states would also need to reengineer no-passing zones to accommodate any degradation in truck acceleration during passing to maintain the current level of safety.

Aerodynamic Effects

Truck-generated splash and spray is sensitive to vehicle aerodynamics. Another aerodynamic effect is the buffeting of adjacent vehicles from air turbulence. Air turbulence around trucks is not increased with truck length or weight, but rather the front of the truck and gaps between the tractor and the semitrailer(s) it tows can be the source of a transient disturbance to adjacent vehicles, especially if they are operating in substantial crosswinds. Double-trailer combinations have two of these gaps, while triple-trailer combinations have three.

As previously discussed, the thrust of the Western Uniformity Scenario is to harmonize weight limits in the western States where LCVs are already allowed. The impacts of aerodynamic effects would not be as much from LCVs being allowed on additional roadways, as it would be from the increased VMT of LCVs and the increased exposure of other vehicles to LCVs. States might consider weather related restrictions on LCV operations, or examine existing ones for revision, if the Western States were to proceed with harmonization.


As with aerodynamic effects, most impacts related to offtracking will be due to increased LCV VMT and not to the introduction of new vehicles. Offtracking measures how well a vehicle "fits" the dimensions of the existing highway system. There are three different types of offtracking that measure the configuration/roadway fit. They are: (1) low-speed offtracking; (2) high-speed offtracking; and (3) dynamic high-speed offtracking. Low-speed offtracking occurs when a combination vehicle makes a low-speed turn - for example at a 90-degree intersection - the wheels of the rearmost trailer axle follow a path several feet inboard of the path of the steering axle. If excessive, this phenomenon may force the truck to swing wide into adjacent lanes to avoid climbing inside curbs or striking curbside objects. Excessive offtracking can disrupt traffic operations or result in shoulder or inside curb damage at intersections and interchange ramp terminals.

High-speed offtracking is the swing out of the rear combination vehicle going through a gentle curve at high speed. Dynamic high-speed offtracking is a swinging back and forth due to rapid steering inputs.

Although these measures relate to a vehicle's operations with traffic, a full discussion of offtracking is presented in Chapter VI, Roadway Geometry, since the roadway curves and intersections dictate how well a vehicle performs.

Assessment of Scenario Impacts

It is not possible to definitively estimate the impacts of the policy scenario on traffic, Level of Service, highway user delay, congestion costs and safety; however these issues can be qualitatively discussed. The CTS&W Study Volume III presented quantitative estimates for the congestion impact for each scenario, but unfortunately, the congestion model is not applicable to the Western Uniformity Scenario because the model does not allow for analysis at less than a national level. The model uses the aggregate national delay derived using PCE values, traffic counts and roadway capacity. The model then applies changes in VMT for the alternative configurations' PCE values to estimate the change in delay.

Also since the CTS&W Study Volume III, there have been changes to the FHWA congestion estimation technique. The new, more empirical approach measures the delay in 75 urbanized areas during peak travel periods as developed by the Texas Transportation Institute. To appropriately apply the urban delay data to changes in the scenario's VMT one would need to determine the number of trucks traveling through the urbanized areas during peak travel times. This is difficult since most long-haul trucks try to avoid city centers at peak travel periods and may entirely avoid urban areas enroute from origin to destination.

Table VIII-3 gives some estimates for the congestion among the analyzed States. Both the Seattle-Everett and Portland-Vancouver areas rank among the 10 most congested urban areas in the country.

Table VIII-3
Percent of Congested Travel for 13 Analyzed States, Year 2000
Urban Area Percent of Travel that is Congested in Peak Period Percentage of Daily Travel that is Congested
Seattle-Everett, WA 79 39
Portland-Vancouver, OR, WA 76 38
Denver, CO 75 38
Las Vegas, NV 65 32
Tacoma, WA 62 31
Salt Lake City, UT 51 26
Colorado Springs, CO 38 19
Eugene-Springfield, OR 33 16
Kansas City, MO-KS 30 15
Salem, OR 30 15
Tulsa, OK 29 14
Spokane, WA 26 13
Boulder, CO 24 12

Source: 2000 Urban Mobility Study, Texas Transportation Institute.

Figures VIII-1 and VIII-2 show that, without any change to truck size and weight, congestion is projected to grow for the Western Uniformity States. This is especially true in coastal Washington and Oregon, Denver and I-80 through Wyoming and Nebraska. It is noteworthy that in the Denver and Seattle/Tacoma areas long doubles are not presently allowed during peak travel times and the scenario assumes that those restrictions would continue. However, because of the shift of some freight to the more productive scenario trucks, thereby reducing total truck VMT, even with these exceptions, one would expect a slight decrease in delay for the 13 States under the Uniformity Scenario. In fact, it appears that the scenario is predicted to at least not degrade and perhaps even improve traffic operations in a small way across all impacts. However, for some of the impacts, this is based on the assumption that increased engine power is available for those configurations with increased gross vehicle weights. Table VIII-4 summarizes the results.

Table VIII-4
Western Uniformity Scenario Traffic Impacts
Impact 2000
(base case)
Traffic Delay (million vehicle-hours) National Total 3,599* Small decrease
Congestion Costs
($ million)
National Total $67 billion*** Small decrease
Low-Speed Off-tracking   Degradation (28 - 30 feet** for turnpike double versus 16 feet for semitrailer)
Passing   Requires operating restrictions.
(merging and hill climbing)
  Requires sufficient engine power.
Lane Changing   Some degradation due to additional length. (This is counterbalanced by decrease in heavy truck VMT.)
Intersection Requirements   Some degradation due to additional length. (This is counterbalanced by decrease in heavy truck VMT.)

*Computed by Texas Transportation Institute as the aggregate for 68 urban areas (not comparable with Comprehensive Truck Size and Weight Volume III).
**28 feet off-tracking for twin 45-foot TPDs and 30 feet off-tracking for twin 48-foot TPDs.
***Estimated for 75 largest urban areas.

Figure VIII-1 Truck Volumes, Estimated Congested Segments – 1998

Map of western states showing truck volumes, estimated congested segments, 1998

Figure VIII-2 Truck Volumes, Estimated Congested Segments – 2020
No Change in Size and Weight Regulations

Map of western states showing truck volumes, estimated congested segments, 2020.  No change in size and weight regulations.

37 See Chapter II, Figures II-5 and II-6 for current weight and length limits in the study States.
38 See Chapter II, Figures II-9 and II-14 for Rocky Mountain Double base case and scenario networks.

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