Longer combination vehicles and the initiative to increase the allowable weight on the interstate highway system has been proposed for many years by primarily large carriers and the Chamber of Commerce on behalf of shippers.  LCVs and heavier weights have always been seen as a way to relieve the driver shortage but recently have been touted as a way to improve safety and reduce the carbon footprint of large diesel trucks.  In the name of efficiency the idea is to move more freight with fewer drivers and move more freight while paying the drivers the same as pulling less weight and freight. 

 

First the people that are supporting LCVs often state that the statistics don’t show that LCVs are involved in any more accidents than other CMVs and maybe even less.  Very vague as there are very few states that allow LCVs and there is no good measurement that has ever been done on LCVs.  Basically there is no real hard evidence in either direction.  That being said the proponents of LCVs always use as their arguments:

• LCVs are more productive due to an increase of cargo-carrying capacity.  Therefore fewer truck trips. Lower cost and fewer miles driven.  It is difficult to argue with this point because obviously there is more cargo-carrying capacity. There is one definite concern here that is never alluded to by these groups and that is about 20% of all miles driven by trucks are empty miles, so by having more capacity doesn’t mean that capacity is used.  As I will mention later the cost of running an LCV to another location (Deadheading) to pick up a load is more expensive.
• Cost:  Fewer drivers needed and more efficient use of fuel.  While this may be true, assuming that all the cargo is going to the same location.  If there has to be several stops or the trailer is not weighted out or cubed out that would not be true.  In fact the opposite would be true.    LCVs need bigger engines with lower end power (Torque) and this is where the greatest amount of fuel is used it is also where the greatest amount of GHG is produced.  In other words if an LCV has to travel to more than one stop or is not fully weighted or cubed it is using more fuel and producing more GHG than a single truck or possibly two trucks.  The wind drag on a truck and trailer is the most significant source of fuel use along with rolling resistance of the tires when traveling at highway speeds.  LCVs use a lot more fuel per mile than a single truck or possible two trucks and produce much more GHG.
• Traffic:  May result in fewer trucks on the road.  But they may be more of a safety hazard as they move slower, require more time to pass and be passed.  Because of their inability to accelerate they often back-up traffic at interchanges and they require greater distances to merge with traffic causing all kinds of maneuvering by other traffic.

 

Now let’s look at some things rarely mentioned that are problems with LCVs.

 

• One of the major safety problems is off-tracking.  All combination trucks experience off-tracking to some extent.  As a tractor trailer transverses a curve the path of the front wheel and path of the rear inside wheel are different. When tractor trailers turn in an intersection the outside wheels of the truck take a wider path than the inside wheels.  One of the most difficult things to learn when driving a CMV is how to turn in an intersection without going into the adjacent on-coming lane.  The driver on urban or rural roads especially has to swing out of their lane to avoid going over the top of a car that may be parked in the adjacent on-coming lane.  This action is a function of the wheelbases of the tractor and trailers and the number of articulation points.  In low-speed off-tracking the rear wheels track inside the path of the front wheels.  In high speed off-tracking the rear wheels off-track outside the path of the front wheels.  There is a definite safety factor when the wheels of the trailer or trailers go into an adjacent lane or shoulder of the road.  Ask any road maintenance crew or city maintenance crew about how often they have to repair curbs and shoulders at intersection used by combination vehicles.  These types of maintenance repairs will undoubtedly escalate if LCVs are more common. 

• In an AASHTO survey based on off-tracking and length characteristics, researchers found that fewer than half of urban and rural interchanges could handle 48 foot tractor trailers. This number “decreased dramatically” for LCVs.  With current designs less than 25% of urban or rural interchanges could handle turnpike doubles.

Here are the percentage of Interchanges determined adequate for various truck configurations by the State Departments of Transportation:

 

	Rock Mountain Double	Turnpike Double	Triple
Rural:	27%	23%	23%
Urban:	29%	24%	39%

 

Can you imagine the cost to the feds, states, cities and towns to widen all interchanges or constantly repair curbs and shoulders!

 

Another cost seldom if ever mentioned is that almost all rest areas would have to be refigured and longer ramps into and out of the area added as the LCVs require greater distance for parking (You don’t back up LCVs) so straight ahead parking is the only option.  I would also suspect there would be more parking on off ramps and on-ramps which would require wider and stronger reinforced concrete to accommodate the weight and length of the LCVs. 

In the study done by AASHTO demonstrated that some truck combinations when confronted by a 90-degree intersection with a 45-foot curb radius (normal) may “encroach into aadjacent lanes on existing or receiving leg of the intersection.”  One study showed that Rock Mountain doubles and turnpike doubles would be forced into opposing traffic lanes far more often than semitrailers to avoid running over curbs when negotiating right turns at intersections.

 

Stability:

 

LCVs are more likely to rollover and are subject o trailer sway and rearward amplification.  The trailers length and the number of articulation points have the most effect on stability. 

• An LCV is more likely to be involved in a rollover because of the nature of the connections used between the tractor and the second and/or third trailer.
• Trailer sway has been shown to be a significant problem for triples which can sway up to one foot and encroach into adjacent lanes.
• Rearward amplification occurs when the truck makes a sudden steering movement.  

Braking and stopping distance: (Remember how ATA used the braking distance to defend the use of speed limiters)

• Generally with properly adjusted brakes the LCVs have more potential braking capacity than conventional semitrailers.  Fact:  In Maryland and California, surveys revealed that half of all air-braked vehicles inspected had at least one brake out of adjustment.  FMCSA stats always show that brakes out of adjustment is the most commonly cited vehicle citation.  
• In sudden braking conditions there could be problems
• The percentage and length of a downgrade can present safety concerns.  Because of the increased weights, LCVs typically must rely more on their brakes.

 

Speed and Acceleration Safety Concerns:

 

• The increased weight means that in order to operate efficiently the horsepower, engine torque, and drive-train must operate without speed differentials among other vehicles on the roadway.  “Crashes are more likely when LCVs travel under the prevailing speed.  For example, when a tuck travels 16 km/hr (10mi/hr) under the prevailing speed, the likelihood for a crash increases by 3.7 times.”  If 20mi/hr under the prevailing speed, the chance of a crash goes up by a factor of 15.5.  
• Trouble accelerating and merging with traffic on freeways
• The LCVs lower acceleration speeds, coupled with its need for more longitudinal space, may also affect the roadway capacity.

As far as weight and bridges, it doesn’t matter if there are more axles to support the added weight when on a bridge the entire weight is on the infrastructure and 100,000 lbs is heavier than 80,000 lbs!

 

The bridge formulas in all the states would prohibit most LCVs as the design of the bridge is such that that much weight is too heavy.  Every state would have to re-evaluate their bridge formulas.  Bridges that have collapsed are still our strongest argument. 

 

There are so many variables involved with pavement fatigue that the weight of the truck or more accurately the weight distribution on each axle is somewhat irrelevant.  It is true that pavement deterioration accelerates with axle weight, the number of axle loadings and the spacing within axle groups. 

 

In addition pavement failure is dependent on numerous variables, including climate, environmental factors, materials, design, traffic and usage.  Most interstates are designed with a flexible pavement that is deeper and design to handle the 80,000 lb loads whereas the arterial roads are usually designed to handle loads of approximately 58,420 lbs.  Since pavement damage increases with time, it is virtually impossible to pinpoint any specific illegally overweight truck to quantify its individual contribution to such damage.  This was all determined by AASHTO many years ago that new pavement construction costs is allocated based on the relative ESALs of each vehicle class.  The ESAL is a measure of the relative contribution to pavement wear associated with different single and tandem axle loads, using an 18,000 pound single axle as the benchmark.

 

Cracking and/or joint-related problems create rigid pavement failure. 

 

Axle groups, such as tandems or tridems influence pavement load distribution.  These groups allow greater weights to be carried and the result in the same or less pavement distress than that occasionedsingle axle at a lower weight.  Now, that is for pavements however, it is different for bridges as the more weight no matter the axles or spacing is a huge problem. (Interesting point here is that wide-base single tires being pushed by the EPA and SmartWay lack strong rut resistance and tend to cause 1.5 times more rutting than dual tires on the flexible pavements.)

 

Interstate bridges are designed to a standard called HS-20.  Most bridges not on the interstate are HS-15 or lower.  Vehicle gross weight, the weight on various groups of axles, the distance between axles, and the type and length of bridge all influence the impact of truck and weight policies on bridges.

 

Pavement costs are dependent on materials, thickness, quantity and quality.  Heavy axles cause greater and faster pavement fatigue. 

 

Pavement damage estimation:  The TRB reported that they and others encountered difficulties when seeking to obtain information about the cost and benefits of truck transportation and the impacts of the size and weight regulations. 

 

Concerning the fourth power of the load:  It was determined in a study done in the ‘50s  that “[t]he relative damaging effect of an axle is considered to be approximately proportional to the fourth power of the load”.  If you double the weight of a vehicle, then the damage it does gets doubled four times.  This means that double the weight causes 16 times the damage.   Put another way an increase in axle weight of 10% will increase pavement damage by about 46%.

 

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