Assessment of heavy trucks on Texas highways: An economic evaluation

ASSESSMENT TEXAS HIGHWAYS C

MICHAEL

OF HEAVY TRUCKS ON AN ECONOMIC EVALUATION?

WALTON,JAMESL

DOCK BURKEQ

BROWNS

Department

+Texa\ Transportation

of Clv11 Engmeermg, Umverslty of Texas at Austm, Austin TX 78712 U S A fThe Asphalt Institute, Austm TX 78712. U S A Institute Texas A & M University. College Station TX 77843

U SA

(Recerwd 8 March 1979) Abstract-This paper describes a study undertaken to assess the effects of projected truck traffic on the highway system of Texas The study Included the evaluation of the costs and benefits for a 20-yr planmng horizon The study wds orgamzed mto thrte phases (a) Current and future truck traffic dlstrlbutlons that wdl most likely occur on the state hlghway system were established for each of two scenarios The first, assumes the present law of a gross weight of 80 000 lb and the second, a possible future legal weight increase to a gross vehicle weight 120,000 lb (b) A evaluation WIS made of the comparative costs required to perpetuate the state highway system m an acceptable condltlon under both scenarios The basis for this evaluation was the general finding from the AASHO Road Test (c) An evaluation was made of the Incremental benefits (associated with the scenarios) defined as increased payloads

CURRENTANDFUTURETRIJCKUSE OFHIGHWAYS

This part of the paper describes two major components of the research (1) review and evaluation of prior studies, literature and pertinent material, (2) ldentlficatlon of alternative scenarios considered to be reasonable posslblhtles from which 20-yr forecasts could be made of truck use of Texas highways Scenat to selec tlon

The ldentlficatlon of alternative scenarios that served m the development of the 20-yr forecasts was accomphshed through sessions of analysis, dlscusslon. and evaluation of both the existing weight hmlts and those used m a Federal Highway Admmistratlon Study (Wmfrey. 1968) as well as those that might be likely candidates for the near future It was decided that, for this limited study, the mdlcatlon of the effects of increased truck weights could best be evaluated by consldermg two scenarios Scenario A would include the continued apphcatlon of existing law on weights and sizes Scenario B would include increasing the maximum allowable weights to those studied by the Federal government (Wmfrey, 1968) but would retam the present restrictions on the size of vehicles 9 tThe research from which this paper developed was sponsored by the Texas State Department of Highways and Public Transportation The materials presented herem are the responslblhty of the authors and not of the sponsoring agency IFormerly of the Texas Department of Hlghways and Pubhc Transportation Wcenarlo A gross weight 80,000 pounds, tandem axle 34000 lb, single axle 20000 lb Scenario B gross weight 120 000 lb, tandem axle 44,000 lb, single axle 26,000 lb 425

Four different types of trucks were selected as the most representative of the existing and future fleet of trucks that will be operating on Texas highways Schematic diagrams of each of the vehicles, along with the maximum legal gross vehicle weight (GVW) and axle loads consldered for each of the two scenarios, are shown m Fig 1 The followmg vehicular dimension restrictions were considered apphcable for both scenarios and are based on existing law Maximum length 45feet for smgle umt 65 feet for trailer and semi-trailer combmatlons Maximum width 96 inches

trucks

Under scenario A, using the exlstmg Texas law, both the axle loads and spacings were restricted through the use of a Bridge Formula which hmlts both axle loads and configurations for the purpose of protecting bridges from excessive damage by loads from trucks (Texas Department of Public Safety 1975) Under scenario B, a similar protection of bridges was provided by keeping the identical axle spacing ProJected

Texas

To facilitate

truck ton-mileage

the forecast of truck types and then assignment to highway classes, a proJection of future ton-miles m the State of Texas from 1977 to 1997 was required The total proJection was dlvlded mto two major categories mterclty and urban The mterclty ton-mileage was allocated to three functional highway classes Interstate, Farm-to-Market Roads. and all other state highways Likewise, the urban figures were allocated to three functional classes of highways m-

426

C

MICHAEL

WALTON.

JAMES

L BROUN and DUCK BURKE

SCENARIO

B

MOX Single AlIt

= 26 000

MarTondcmnrle MO” G Y w AlIe

= 44,000 = m.ooo

cvw (cross

Uelght)

1

Vehicle - 33,000 lbs

@=J I

1

32’

Gw32’

16K

@zJ

I

Gvy - 60,000

I

‘XX-

17’

1,~

2-Sl-2

A&-&--J

~~~~~~~~

I

AXuwWTaK

8’

I

Gyw - 80,000

21’

iaK

I 10’

iaK Fig

1

I

21’

iaK

I iaK

Conzpututlonul procedures To Illustrate the basic procedure, Fig 4 shows how the truck populatmn IS likely to be affected by a so

1

Propsctlon

of

Including

Fig 2 ProJectlon

Texos IntercIty

104,000

Ibs

14’1

34’

44K

44 K

,im-120*ooo1b* I 8’

I

16K 26 K

21’

I

26K

10’

I

21’

26K

I

26K

Selected truck configuratlons for scenarios A and B

terstate. other state freeways and arterlals, and collectors Figure 2 depicts the 1977-1997 forecast, and Fig 3 shows the 1997 allocatIon of ton-mliedge to VdrlOUS highway types The forecasted ton-mileage was assumed to remam constant m both scenarios

80

lbs

14’1

Ibs

14’1

28’

A&&

18

lb*

26K

cw-k7’0001bs

288 141

I

k2’ooo

I

Truck - Urbon

change m the maxlmum legal GVW First of all, there ~111be more trucks operating above the current legal hmlt (80,0001b) These ~111 be replacing some trucks thdt had been operating near and below the old limit As a result of the shift, there will be an overall reduction in the number of loaded vehicle trips dnd. correspondmgly, d decrease in the number of empty trips At the same time, there ~111 be a portion of the truck population that ts unaffected by the change m maximum legal GVW The loads on these trucks are either low density commodltles (volume constrained) or partial loads (demand constrained) The procedure used data collected by the State Department of Highways and Pubhc Trdnsportation (SDHPT) over the past 20 yr (Texas Hlghwdy Department 196%1971, 19731975) The data represents vehicle (empty and loaded) weight Intervals sampled at designated highway locations around the State The mformatlon, therefore, 1s classified by highway classes and vehicle types m addition to weight mformatlon The dlstrlbutlon of gross weights for specific

Ton-Mlleoge Split

of Texas truck ton-rmleage mterclty-urban spht

mcludmg

Fig 3 Summary dllocatlon chart for final 1997 Texas truck ton-rmledge (m Mhons of ten miles)

Assessment

of heavy trucks

FUTURELAW

Gross

Fig

4 Truck

The

from

Weaght _

populations and changes resulting Increase in mdxlmum legdl GVW

classes of trucks lished

Vehicle

under

the above

process

required

existing

legal

mformatlon the

limits

from dn

was estab-

t

development

of a tech-

nique for computmg the average empty vehicle weights, the average payload carried, and the 18,000 lb single axle load (KSAL) for each vehicle type and each hlghway system The number of 18 KSAL, truck operatmg costs, and fuel consumption for each highway class for each year over the forecast period (20 yr) were calculated by using the truck freight ton-mile allocation for each class, the average payload per mile of a system for each year, and the total number of vehicles required to carry the freight allocated to that vehicle type The process was repeated for each of the two scenarios The 18 KSAL output was used as input mto SDHPT programs for computmg the impact on highway maintenance and rehabilitation The truck operating costs and fuel consumption data were used as input to the evaluation of benefits The actual procedure used m the computations was obtained from a national study of truck sizes and weights (WhIteside 1973) This Natlorral Cooperative Highway Research Program (NCHRP) model was modified and adapted for use m the present study Modtjicatlons of the NCHRP methodology Initial efforts m this study utlhzed the NCHRP 141 methodology with only those minor modlficatlons *The avallable GVW dlstrlbutlons for Texas hlghways were consldered to be representative of trucks operatmg before the last mcrease m maxlmum legal weights had occurred (72,000 GVW max to 80,000 GVW max ) Therefore, It was necessary to shift the GVW dlstrlbutlons from the previous law to the present law before the computations reported herem were conducted

on

TeXdS

427

highways

necessitated either by absence of required data or a different form of available data However, an examination of the prehmmary estimates of costs and benefits led to a more extensive critique and modlficatlon of the basic NCHRP 141 method In order to predict that will happen to the dlstrlbutlon of gross vehicle weights for the various types of trucks after a law change, the NCHRP 141 researchers exammed hlstorlcal GVW dlstrlbutlons before and after size and weight law changes A pattern existing m this data shows a shift to heavier trucks with a small shift on the empty weight portion of the dlstrlbutlon A shift approximately proportional to the ratlo of the practical maximum gross weight under the new law to the practical maximum gross weight under the old law exists on the loaded weight portlon of the dlstrlbutlon Figure 5 illustrates this trend These hlstorlcal shifts were modeled by a shift that started with no change for the smallest vehlcle Increasing proportionately to a shift equal to the ratio of practical maxlmum gross weights mentioned above The results of applymg this type of shift to 100 vehicle miles of scenario A of 3-S2 trucks are shown m Fig 6 Figure 6a shows a large decrease m 18 KSAL for trucks operating near the present legal hmlt This decrease 1s negated by the increase caused by the new heavy trucks Figure 6b IS slmdar except that a large savings m truck operating costs 1s mdlcated for empty and lightly loaded vehicles Such data cdused the authors to re-examme the shlftmg procedure One might expect the followmg to happen If weight (only) laws were changed (I) Those trucks operating near the legal axle or gross weight limit would increase their loads This might or might not require a heavier vehicle Fewer trips would be required to carry the same freight for this group of vehicles Fewer empty or return trips would be required (11) Some vehicles carrying low density cargo are constrained by the volume (size) of their vehicles loo-

/ D,strlbutlon Before Low Changes -‘r/T/’

60 I I--’

i ,o f 60 -

I /’ I-,

2

Gross Vehicle

Fig

5 Typical

/ ~trlbutmn

historlcdl

1’

wth Increased Weight Lows

Weights

shifts m gross weight dlstrlbutlons

C MICHAELWALTONJAMESL BROWNand Dock BURKE

378

Gross 40

Vehicle Weight, KIPS 60 80 100 120

NCHRP

20

40

60

Gross

Vehicle

60

100

Weight,

120

5_

&Y

z.m+ -

2”

IO20

&C

60 60 100 120 Gross Vehicle Weight, KIPS-

140

,,,

Pi c.3

40

140

SHIFT

KIPS -

‘;v,

51 20

/4/

5-

40 Gross

60 60 100 120 Vehicle Weight, KIPS-

140

Fig 7 Change In KSAL \s GVW

These vehicles would be unaffected on both the loaded and empty trips (111)A slgmficant number of trips are made where the vehicle 1s only partially loaded Some of these are delivery trips wherein the weight decreases or mcreases along the route Segments of these trips may be affected by the law change while the less loaded trips are made because the demand IS only for a partial load These trips n111 be unaffected by the law change It was concluded that a shifting procedure would be used that would have the foliowmg characteristics (1) Heavily loaded vehicle trips would shift to d larger GVW m proportion to the previously mentioned ratlo of practical maxlmum gross weights (11)Lightly loaded vehicles would be unaffected by the law change (111)Empty vehicle trips would be reduced m proportion to the reduction of loaded vehicle trips It 1s postulated that the historical changes m GVW dlstrlbutlons used as a basis for the NCHRP shift were the result of factors other than weight law changes It 1s possible that trucks were becoming heavier with time. and that concurrently with weight law increases. size increases were also permitted To explore this phenomenon, a sensltlklty study was conducted exammmg the effects of several possible shifts on the computed savings m truck operatmg costs and increased 18 KSAL In general, truck operating cost savings are more sensitive than 18 KSAL are to shifts that increase weights of lightly loaded trucks Further, for shifts that affect primarily heavily loaded vehicles neither output 1s extremely sensitive to the shifting procedure

The results obtained with the shifts are illustrated m Figs 7 and 8 Note that for the adopted shift (SDHPT shift) the followmg results were obtained (1) Fewer empty trips resulted m savings (11) Some partially loaded or lightly loaded trucks were unaffected (111)Trucks possibly constrained by axle or gross weights laws were reduced (iv) Trucks exceeding the present law (but constrained by the future law) were increased This resulted m increased savings (v) Net savings m truck operating costs were affected much more than the net increase m 18 KSAL by the adopted shift versus the NCHRP 141 shift Gross 20

40

Vehicle Weight, KIPS 60 60 100 120

140

IOr NCHRP

I41 SHIFT

L

Fig 8 Change m truck operatmg costs vs GVW

Assessment

of heavy trucks

429

on Texas highways

zoning until replacement if the load hmlt laws were mcreased For this study it was assumed that the scendrlo B law Increase would be effective m 1980 As a result of the load restriction on bridges it was estimated thdt d 14-yr program of bridge replacements would be necessary to implement fully scenario B It was further estimated that a selective replacement scheme could be devised so that 90”, of the affected freight haul demand could use the system wlthm 8 yr, while the remammg ten percent could use the system by the end of I4 qr The highway costs and the truckmg industry benefits were phased m using this 14-y’ transltlon period beginning m 1980

~~&_+27.‘“TRuc~) 0

4

20

40

60

80

0

: c

;

15

3A

t

z_ z

0

0

20

40

60

TRUCK

80

w

2 B

2

15

3-52

t 2 I a B

_s NCtiRP

0 O

0

20

20

I30

HIGHS AY COSTS 40

40 G”W

Fig 9 Multlphers

TRUCK

141

I” K,ps

60

60

60

80

-

adopted for shlftmg GVW dlstrlbutlons from scenario A to B

Figure 9 shows the NCHRP 141 and SDHPT shlftmg factors The SDHPT shift 1s considered a “most likely ’ outcome, however, it must be pointed out that the basis for its selection lacks preclslon The followmg quotation from the letter of transmittal for Wmfrey, Howell and Kent (1976) 1s given to show how uncertain present knowledge IS on this subJect

“Consldermg the fact that local gross weight hmlts have increased steadily during the past two decades, the major findings that there has not been a marked change m total truck traffic nor m average gross or axle weights during the period 19661972 are surprlsmg This result may be a reflection of trends m actual gross weights due to more haulage of manufactured goods or may be due to avoidance by truckers of fixed weighing station locations ” For much cargo, the point of dlmmlshmg returns as far as gross or axle weight hmltatlons are concerned may have already been reached Implementatron of scenarios A and B At present, there are approximately 24,COOmiles of farm to market (FM) Roads and 900-plus bridges load zoned to less than the vehicle weights considered for scenario A However, It was considered more reasonable to Implement scenario A as If no restrlctlons existed since enforcement IS difficult A comparison of the weights of trucks proposed m scenario B with the allowable truck weights on the bridges on the current network of highways revealed that a significant number of the bridges would reqmre restrlctlve load

The general approach used m estlmatmg mcremental highway costs to accommodate the heavier trucks of scenario B wds to estimate the costs of only those items of hlghway maintenance and constructlon that would be affected by the heavier trucks Costs to mamtam the existing network of pavements m good condltlon for the next 20 yr were first estimated (scenario A) A second estimate was made assummg that the gross weights and axle weights were increased (scenario B) Included m the pavement costs were the costs of routme pavement maintenance, seal costs, and pavement rehablhtatlon Also included were the estimated costs of upgrading current structurally deficient bridges to carry the loadmg of the two scenarios The cost estimates do not include structure maintenance, nor bridge rehablhtatlon and replacement due to functlonal deficlencles and deterloratlon These costs were excluded due to the mablhty to Isolate bridge maintenance requirements dssoclated with heavy loads and the lack of current technology for dnalyzmg the effects of repetitive hedvy loadings on the life of structures Although evaluation technology 1s not available, it 1s known that heavier and more frequent loads will accelerate wear-out of bridges Because pavement deterloratlon IS caused by both truck loddmg and environmental stresses, the routme maintenance and seal coat costs were dssumed to remain constant m both scenarios This assumption implies that routme mamtenance and seal coats are sufficient to handle the envlronmental deterloratlon Pavement rehablhtatlon costs were estlmated to increase with the heavier trucks The resulting annual and cumulative cost estimates are shown m Figs 10 and 11 No data were available to estimate the costs of roads and streets off the state system, however addItiona costs would be incurred by cities and counties to handle the larger trucks on the city streets and county roads Table 1 contams the costs accumulated for the analysis period Other hlghwav construtlon coyty The largest highway construction costs, those required for reconstructmg pavements due to accelerated wear-out and those associated with replacing

430

C

MICHAEL

WALTON,

JAMESL

BROWN

and DOCK

FISCAL

BURKE

YEAR

5

78

Fig

66 FISCAL

10 Costs to mamtam

98

YEAR

the exlstmg

system (Mamtamence

replacement

and rehablhtatlon)t

tBrldge costs Included m total5 only reflect expense of upgradmg structurally deficient bridges to carry the loadmg of the respectwe scenarios Not mcluded are the costs of bridge maintenance, rehablhtatlon and replacement due to functional deficlencles and deterloratlon

Cortfor Seenarm A

‘;ye;Fts

FIN

I1 T&enty-yedr

Total state Hrys

IH HWy?i

FM Roads

(71.000)

(3,COO)

(41,ooO)

Other state Hwyr

city streets

COW Road!

(27,000)

(55,OGO)

W5.000)

cost (1977. 1997) to mamtam

exlstmg

systems.7

tBrldge costs Included m totals only reflect expense of upgradmg structurally deficient bridges to carry the loadmg of the respectwe scenarios Not mcluded are the costs of bridge maintenance, rehablhtatlon and replacement due to functional deficlencles and determmatlon

Table

Assessment

of heavy trucks

1 Comparative

twenty-year

Interstate highways

Scenario A Pavement mamtenance Pavement rehablhtatlon Bridge replacements* Scenario B Pavement mamtenance Pavement rehabilitation Bridge replacements*

& seal coats

on Texas hlghways costs for scenarios

431 A and B*

Farm-tomarket roads (Mllbons of constant

Other state highways 1977 dollars)

Total state system

Totals

$240 1334 4t $1578

$1100 1512 76t $2688

$960 3084 50t $4094

$2300 5930 130t $8360

Totals

$240 1888 172t $2300

$1100 1953 376t $3429

$960 4618 554t $6132

$2300 8459 1102t $11 861

& seal coats

tBrldge replacement costs include only the estimated cost of upgrading existing bridges to carry the loads included m the two scenarios The cost of structure maintenance, bridge replacement and rehablhtatlon due to functional deficiencies and wear-out are not included because of the mablhty to Isolate structure maintenance requirements associated with heavy loads and the lack of current technology for analyzing the effects of repetitive heavy loadmgs on the life of structures Therefore the totals do not reflect the entire cost of mamtammg the existing system

load-zoned bridges, have Increases that are smaller Incurred Estimates for made because of either data Some of these costs gorles

been estimated Other cost but still slgmficant ~111 be these costs have not been time hmltatlons or lack of fall mto the followmg cate-

(I) Costs of replacing bridges and pavements on county roads, city streets, private driveways, and parkmg terminals (11) AdditIonal costs of maintenance m deslgnmg new bridges and pavements to accommodate the heavier loads (111)Costs to provide somewhat flatter grades or more chmbmg lanes If proportionately more horsepower IS not provided with the heavier trucks (iv) Costs to provide safety features m hlghways, If superior brakmg systems are not provided (Improvmg the exlstmg highway network to accommodate such trucks to current “safety levels” might prove to be very costly Among the features possibly needing change are sight distances, median widths. guardrails, and median barriers )

the FM system, and the All Other category Note that there are many non-Interstate lane-miles that have not been rehablhtated or reconstructed m the last 20 yr Pavement rehablhtatlon proJects vary from simple, thm overlays to maJor reconstruction As a general rule, thm overlays will purchase addltlonal hfe economlcally. but these can be applied only a few times to an exlstmg structure before reconstruction becomes necessary From the experience of recent years, a reahstlc mix of such thm overlays and reconstructlon projects was estimated for each highway system and each pavement type considered m the REHAB program Costs for each type of rehablhtatlon proJect

4000 2000

Inlerrtaic HIghrays 19,969 ‘.a”, Ame.

h ookk=e Age as of 1976

Busts for pavement cost eytwnates A computer program entitled REHAB, orlgmally developed m the McKmsey and Co (1976) study, was improved and used to project the pavement rehablhtatlon costs Inputs to this program include the number of lane-miles of pavement, their age, unit costs for rehablhtatlon, and survivor curves which portray the expected life of the pavements The lane-miles and aget data were obtained from files mamtamed by the SDHPT Figure 12 shows the age dlstrlbutlons of existing lane-miles on each of the state systems The most recently constructed pavements have been on the Interstate system, followed by

Aga

of 1976

State

Age

t”Age as defined in this data IS the time elapsed smce constructlon reconstruction, or rehablhtatlon

OS

Fig

as of

12 Age dlstrlbutlon

Htghroyr

1976

of existing

lane miles

C MICHAEL WALTON JAMES L BROWN and DOCK BuahF

432

Table 2 Comparlaon 18 000 No equlvdlent axle Iodds per 20 jr Scenario Scendrlo B A Interstate lughua?s Flexible pavement RigId pavement Farm-to-market roads Flexible pavement RIgId pavement Other state hlghways Flexible pavement Rlgld pavement tPavement

7 813,000 12 980,000 92,800 141 100 871700 1 308 000

hfe before reconstructlon

of REHAB

Inputs

Ratlo of pavement hfeet m scenuo B to \cendr1o 4

11,720 000 20 250 000

0 667 0641

1 076 1027

194,800 278,800

0476 0 506

1 065 1057

1602 000 2 435 000

0 544 0 537

I 044 I 044

to accommodate

heabler trucks After reconbtructlon the rdtlo equdla one

were estimated usmg unit prices prevalent m 1977 A proportlonate mix of mmor and mador rehablhtatlons was used as input to REHAB to represent the proportions of minor and malor rehablhtatlons thdt are most Iihely to occur Survivor curces showing the percentage of each pavement type that 1s expected to surklve to a certam age were estimated by a panel of experienced pavement engmeers for use by McKmsey and Company when the original REHAB model was developed These curves were updated for this study using mformatlon made available to the panel subsequent to the orlgmal estimate This new data conslsted of survivor curves for a sample of pavements obtamed both from a research project bemg conducted by the Texas Transportation Institute (undated) and from some survivor curves from an older (Texas Hlgh&day Department 1973) Planmng Survey Study It wds necessary to devise a method for adjusting the pavement hfe when either an Increase m truck volume or heavier trucks are operated over a road segment This adjustment procedure was based on the results of the American Assocldtlon of State Highway Officials (AASHO) Road Test (Hlghwdy Research Board, 1962d) The expected pavement libes me the survl\or curves, were shortened m proportion to the increase m equlkdlent axle loads supphed from the proJected traffic discussed previously It \hds also necessary to mstltute this addltlonal aging of the pavements at the expected time of occurrence of the heakler trucks To Illustrate this concept, suppose the total expected life of a particular pavement m scenario A IS 10 yr and It has 4 yr of remammg hfe Further, suppose that the heavier trucks of scenario B would double the rate of damage to the pavements based on the equlkalent load concept mentloned above With the change m truck loadmg. the remammg hfe of the road would then be only 2 vr Another revlslon to REHAB was necessary Followmg the accelerated wear-out of the exlstmg pavements, it would be desirable to re-design pavement structures to handle properly the heavy trucks The program was reklsed to accomphsh this for that portion of the pavements recelvmg major rehablhtatlon

The orlgmal sur\Ivor curves (those developed under more recent weight standards with longer h\es) were then applied to these pavements The increased cost to dccommoddte hedvler trucks ~d\ e\tlmated to be proportlondl to the ratio of the logdrlthm of the heavy traffic equiralencles to the logdrlthm of the orlgmal traffic equlvalencles This methodology IS also based on the findings of the AASHO Road Test (HighRay Research Board, 1962b) Table 2 shows the relative magnitude of the changes made m the Inputs In summary the necessary re\lslon\ changed the REHAB program so that the follonmg operdtions occur (1) When heavter truths dre dpphed the hfe curtes are shortened, causing faster wear-out of the pd\ements (II) The “worn-out’ pavements dre rehdblhtated Those recelvmg mmor rehablhtatlon (thm o\erld]s) contmue to wear out at the accelerated rate However, those receiving maJor rehabihtatlon dre re-deslgned at an increased cost to handle the hedvler truths The\e re-deslgned pavement structures nom begin to wedr out at a slower rate The slower rate 1s the same rate as the orlgmal life curhes for these pavements

Bass for tmdye cost emmatet The Federal Highway Admmlstratlon and Amerlcan Assocldtlon of State Highady and Transportation Officials (AASHTO) have developed d formula for calculatmg a Sufficiency Ratmg for bridges This formula takes mto consideration structural adequacy and safety features servlceablhty and functional obsolescence, and essentiality for pubhc use If a bridge has a calculated Sufficiency Rating of less than 50 It can be consldered eligible for repl‘tcement under the Natlondl Special Bridge Replacement Program Using the above formula and current bridge mspectlon data a Sufficiency Rating wds calculdted for dll bridges on the State hlghwa) system The bridge replacement costs for scendrlo A were developed by applymg the same criteria thdt is used by the Federal Hlghway Admmlstratlon m the National Specidl Bridge Replacement Program and addmg addItIona Iodd-restrlcted bridges Scendrlo B required e\Ju-

Assessment

of heavy trucks

&Ion of the effect of the Increased truck loadmg on bridges This wds performed generally in accordance with the methodology of NCHRP 141 procedure (WhItesIde 1973) From computer hstmgs representmg all of the bridges on the Texas highway system, five types of simple span superstructures and seven types of continuous span superstructures were selected as representatl\e of the entire system of bridges Average span lengths were also assigned to these twelve types Contenuous sp,ms were represented by dn equrvalent slmplespan length The usual ratlo ofdead load moment to hbe load moment wds estabhshed for each type by calculation dnd estimate These vary shghtly from those reported m the NCHRP 141 procedure to correvpond more nearly to Texas condltlons Each structure-type-span was considered for four design loadmgs on each of three hlghway systems (Interstate, FM and all others) Live load moments due to one design truth or lane were taken from the AASHTO Bridge Speclficatlon, Appendlv A, for each span Moments due to one truck from the proposed legal loading, scenario B, were calculated for each span using a computer program called BMCOL 43 (Mattock and Taylor 1968) Trucks represented by scenano B uere considered and the absolute maxlmum moment for the span was used The ratlo of the Scenario B moment to the design load moment represents the increase in h\e load moment for each typespan To convert this to stresses, the dead to h\e load ratlo was used and for prestressed beam bridges only d factor e\aluatmg composite action was included The formula selected for calculatmg overstress IS that used m NCHRP 141 In order to evaluate the effects of the overstress, It was necessary to establish allowable values for the hdrlous types of bridges For structural steel bridges. the steel stress was hmlted to 75”” of the yield stress v,lth capacity reduction factors m accordance with the maximum stress permltted by the American Assoclation of State Hlghway and Transportation Officials (AASHTO) M~~nurrl fijt Mutntenaflte Inspectzoll of Brldgrs for operatmg ratmg For these steel bridges, dn o\erstress of 23”, wa5 considered allowable For concrete bridges, an overstress of 26”. was considered allowable

Whenever the calculated overstress exceeded the allowable overstress all bridges represented by the type-span-loading were considered inadequate for scenario B loads and therefore required replacement Where the olerstress was less than the allowable, the bridges were considered adequate The number of bridges and their deck area were tabulated for each type-span-loading system The deck area of bridges which are presently load restricted was tabulated and subtracted from the total to prollde the bridge deck area that would be affected by the proposed changes for scenario B If the category showed excessive overstress, the cost of replacement was calculated by multlplymg the affected deck

433

on Texas hlghwdys

area by the estimated umt cost of construction Culverts were not considered to be affected because the maximum scenario B wheel loads are less than the present design wheel loads The bridge replacement costs calculated are shown m Table 1

DECREASEDTRUCK OPERATING COSTS AND OTHER BENEFIT

CONSlDER4TIONS

A change m the truck weight limits will produce changes m a variety of costs and benefits associated with the movement of hlghway freight Estimates of some of these changes have been made and are dlscussed m this section In summary, the major quantlfiable effects that cdn be expected under the higher weight limits of scenario B are (1) $9 12 bllhon savmgs m truck operatmg costs, (11)2 42 bllhon gallons of fuel saved, (111) neghgrble changes m noise and air pollution m urban areas Decreased truth

operatmg

costs

The primary benefit obtained by the hypothesized change m the weight hmlt accrues m the form of reduced operating costs m the truckmg Industry The projected snvmgs are shown m grdphlc form m Fig 13 The proJected $9 12 bdhon savings that occurs wlthm 20-yr analysis period (_l977-1997) was calculated usmg a procedure similar to one presented m NCHRP 141 The data base for the operating costs was obtained by updating the cents-per-ton-mile numbers described m NCHRP 141 The components of the total operating costs perton-mile are repalr, servlcmg, and lubrlca:mg costs, tires/tubes costs, fuel costs, driver wages/subsistence costs, overhead and mdlrect costs and depreclatlon and Interest costs Several different cost mdlces were obtained and applied to the NCHRP 141 data to update the operating cost mformatlon from 1970 to current 1977 levels m an attempt to derive a comprehenslve measure of the resulting dollar savmgs The resulting data collected from both public and private sources did not produce a set of compatible mdlces for each of the SIXcost components The general Consumer Price Index (CPI) was finally selected as the mechamsm for updating the 1970 truck operatmg costs Recently published results of a study conducted by the Hertz Corporation suggest that increases m truck operatmg costs since 1975 were larger than those reflected m the CPI The Hertz data, however were not mcorporated m the present analysis due prlmarlly to time constramts The savmgs shown m Fig 13 are probably on the low side due to the relatively more rapld mcrease m fuel costs not reflected m the estimates The proJected ton-miles data were allocated to the three hlghway systems, and wlthm each of the three hlghway systems, the ton-mileages were further allocated to the selected vehicle types As a result, the

434

C MICHAEL WALTON, Interslate

500

JAMESL BROWNand

DUCK BURKE

Hqhroys

t

Total SO0

la the State

System

t

_. 6 =

_5

1FM hods

FISCAL

CityStreets

and County

YEAR

Roods

3

88

78

FISCAL

YEAR

Fig 13 Truck operating cost satings 1978-1997 scendrlo B over scenario A number of ton-miles being hauled by by each vehicle weight class was calculated for each scenario The hypothesized change m truck weight limits allowed the heavier weight class vehicles to haul more of the ton-miles, which resulted m fewer trips and therefore lower aggregated costs of truck operations m scenario B On a dlsaggregated basis, the cost savings by types of systems are also presented m Fig 13 The mam point to be concluded 1s that 50”, of the calculated savings occur on the Interstate system, 42”, on all other highways, and only 8”, on the FM network Fuel savrngs

Due to the current Interest m the energy situation, a separate analysis was conducted to examme what, if any, fuel savings might result from an increase m truck weights From a review of the literature (Hall, 1956, Kent, 1960, Highway Research Board 1961, US House of Representatives, 1965 Bender and Kaye. 1975) the followmg model was selected to relate gallons of fuel per mile (gpm) and GVW gpm = 0 139 + 0 00145 GVW Using

the above equation,

mterclty

ton-mile

fuel

consumption rates were calculated ProJected fuel savings are shown m Fig 14 The fuel saved would be about 1 Sol0 of the amount needed without the mcrease in truck weights The total 20-yr savings-242 bllhon gallons-represents an amount approximately equal to 289, of all the motor fuel used m Texas in 1975 Air and nme pollutm~ ejects Some analyses were completed m an attempt to relate vehicular pollution and changes in vehicle weights The results are derived from previously dekeloped models (Ingalls and Springer 1974, 1975 U S Environmental Protection Agency, 1974) In the three maJor Texas metropolitan areas (Dallas-Fort Worth, Houston-Galveston. and San Antonio), a 3-6”, reduction m air pollution caused by heavy trucks was calculated Since heavy trucks contribute relatively small amounts to the total pollution emitted by all transportation, this calculated decrease represents a less than l”, reduction m transportation generated pollution The akallable data and research on noise pollution indicated that the hypothesized increase m axle weight limits should generate only small increases m

Assessment

I20 t

of heavy trucks

435

on Texas highways

Interstate Hlqhwoys

co-

Other

Haqhways

Slotc

FISCAL

98

78

jGty

Streets

County Roods

an;

78

88 FISCAL

Fig

YEAR

14 Savmgs

,

98

YEAF4

In fuel consumption,

noise along highways Estimates of these reductions were not calculated because of the mcompleteness of techniques m the state-of-the-art (Working Group on Research mto Road Traffic Noise 1970, Staadt, 1975, Transportation Research Board, 1975. Close and Wesler, 1975, US Department of Transportation, 1972, Alan M Voorhees and Associates, 1973, Agent and Rlzenbergs, 1976, and U S Environmental Protection Agency, 1975 ) Other constderatlons

Many significant conslderatlons were involved with both srze and weight changes m truck usage that were not considered explicitly m this study These include, but are not limited, to the followmg (1) Geometric design and re-design and construction of streets and highways to accommodate larger trucks, eg longer and wider vehicles resulting m modlficatlon m lane, median and shoulder widths, passing lanes, turning radu at curves and mtersectlons, signing, safety rest-stops, right-of-way requlrements, etc (11)Highway safety conslderatlons reflecting a more diverse mix of vehicles traveling on the highways, e g

1978-1997 scenario B over scenario A larger, longer or heavier trucks mixed with mcreasmgly smaller automobiles create significant safety issues which may be translated mto higher accident rates and a correspondmg increase m accident sever1tY (m) Other highway operational lmplrcatlons such as wet weather condltlons (splash and spray), overnight vehicles, hazardous loads, etc (iv) Costs of replacing bridges and pavements on county roads, city streets, private driveways, and parking terminals (v) Additional costs of the construction of pavements and bridges to accommodate heavier loads on new locations (vi) Accelerated bridge wear-out related to heavier and increased frequency loadings 1s known to occur but cannot be quantified with current technology (vu) Imphcatlon of new design trucks and performance, such as their acceleration and braking capablhtles, and any modlficatlons m truck climbing lanes and downgrade conslderatlons (vm) Changes m technology m the goods transportation mdustrles (lx) Externahtles associated with heavier truck

C MKHAFL WALTON

476

Table 3 20 yr mcrementdl

JAMES

co\ts dnd savings

AddItIonal hlghaak Costa (In bllhona of constant 1977 dollars)

loads and the freight shdres of rail, plprhnes wateruays due to modal shifts

and

with

costs hedvler

between truck

scenarios

lodds

and

A and the

B

corres-

ponding savings m truck operating costs for the ?O-yr analqsls period were computed for the three hlgh\hdq classes Also included waq an estimate of fuel savings These are shown m Table 3 From the study, It has determmed that if height laa changes are undertdhen. further analysis would be Justified to select those routes that uould cnrry rel+

tlve!y

large

freight

dnd hould

tonnages

cost

relntirel!,

less to upgrade

Also it cdn be been

upgraded

inferred

to handle

Sd\mgb m truck operatmg costs (m bdhons of constant 1977 dollars)

that

once

the hedlier

the highways trucks

Hall W

ha\e

the addl-

tlonal

cost to mamtam the system for the healler trucks will decredse In other herds, the annual addltlondl costs beyond 1997 would be less thdn those annual costs occurring during upgrddlng

REFERFhCES

K R and Rlzenbergs R L (1976) Vehl& nol\e survey m Kentuchq Hzyh,\lri Rewmh Recmd 580 W dshmgton D C Aldn M Voorhees & A%ocidte\ Inc (1973) An dndlysis of the economics of truck sizes dnd weighta I” relation to Prrpdred for Motor state and federal regulations Vehicle MdnufXturers Associdtlon, Detroit MlLhlg‘m Bender E K and Kaye M C (1975) Truck no,Fe III-g Field test of frelghtlmer guided truck Prepared by Bolt Beranek and Newman Inc for the Department of Trdnsportatlon Washington DC Close W H dnd Wesler J E (1975) Veh& noise souizes and nol,e suppression potential Zfr~tor L ehlc /e NOI\L’ Control Transportation Research Board Special Report 152 National Research Council Wa\hlngton D C

1 71 0 18 103 2 42

modern

to the Montana

lughwdys

for Mon-

fact finding committee

on

highways streets and bridges Helena Montana Hlghwaj Research Board (1961) Line-haul trucking cost m relation to vehicle gross weight Hqhti~~l Receurch Bourd Bulkt~n 301 Washington, D C Highway Research Board (1962a) AASHO Road Test Report No 1 HiyhbtuL Re~urch Botrrd Spe~rtrl Report 61A Wdshmgton DC Highway Research Board (1962b) AASHO Road Test Report No 5 Hlyh\tu\ Re$eurch Bourd Spetrcll Report 61E Washington DC Kent hl F (1960) Fuel and time consumption rates for trucks in freight ser~lce Hlghrtul Rrwurth Board Bulletm 376 Washington. D C Ingalls M W and Springer K J (1974) Mass emlsslons from diesel trucks operated over d road course Southwest Research Institute San Antonio Texas Prepared for the U S Emlronmental Protection Agent, Ingalls M W and Springer K J (1975) Mass emlsslons from ten pre-controlled gasoline trucks and comparisons between different trucks on a road course Southwest Research Institute. San Antonio Texas Prepared for the U S Emlronmentdl Protection Agency Mattock. H and Taylor C (1968) A computer program to analyze beam-columns under movable loads CFHR Report 56-4 McKmsey dnd Co (1976) Guide to the Hlghway RehablhtatIon Forecdstmg Model McKmbeq and Co Chlcago Ilhnolq Prlede T (1967) Noise and vlbratlon problems m commercial vehicles J of Sound prh 5 (1) 129-154 Road Rese‘trch Labordtorv (1970) A Reblew of Road Tralfit Noise RRL R~pvrt LR 35’ London England Staadt R L (1975) Truck noise control Redtrctron of Iftrchrncr 1 Now (ed bv J Malcomb) Revised Edition Texas Department of Pubhc Safety (1975) Trwy Reyular~on~ Gmrmrny the S1ze and Welyht c/et Austm Texds

Texas Highwd, M eryhr

Agent

trucha

Fuel sd\ mgs (m bdhons of gallons)

F (19561 Flndncmg

tana d report

The major approach to this study mvolled the estlm&on of the compdrdtive maintenance dnd rehablhtatlon costs of perpetudtmg the state hlghhaaq system under current weight hmltatlons and on future use under different height condltlons The\e costs were based on alternatlle weight hmltatlons on truck use and did not consider size alternatives An increase m the size of vehicles has significant ramlficdtlons beyond the scope of this study and IS mentioned only to endble a better appreciation of the hmltdtlon lmpllclt m these findings incremental

with shift to heavier

4 57 071 3 84 9 13

FINDlrVGS 41\iDCONCI LISION‘,

The

dbbolldted

0 72 0 74 2 04 3 50

Interstate higHwd?q FM roads Other State hIghways Total for hIghway slatema

associated

L BROMN and Doch BURKE

trnd

Depdrtment

I

rhlcle

(196&197l

of CommrrcuI

1973-1975) Federdl

C/UWI~~~MWI St&v

P ehlTruck

Highwdy Admnustratlon Washington, D C Texas Highway Department (1973) Road L$r Srud\ Planmng Survey Dl\lslon Austm Texas (unpubhshed) Texas Trdn~pOrtatlOn Institute Flevlble Pavement Evaludtlon and Rehablhtatlon Research ProJect Z-8&75%207 College Stdtlon Texas (unpublished ddtd) TrdnapOrtdtlOn Resedrch Board (1975) hforor I/ehrc/e Now Control Transportation Research Board Special Report 152 NatIonal Research Council II S Department of Trdnsportdtlon (1972)Trunsportutmn Nose and it\ Conrrol U S Government Prmtmg Office Washmgton DC U S Environmental ProtectIon Agency (1974) lY72 Nrrtlo,ltr/ Ern~~\uln\ Reporr Ndtlondl EmIssIona Ddtd System of the Aerometrlc and Emlaslons Reporting System U SE P 4 Wdshmgton, DC U S En\mmmentnl ProtectIon Agency (1975) Yolye Ne\\

Asseu\ment

of heabv truths

Frdrrul EP 4 Rcyulurum Gorernmy Inrrr\r~rre r\fotor Carrwr~ U S E PA, Washmgton D C U S House of Representatives (1965) Supplernentar~ Report of [he Hqhrvo~ Co\t 411o~utum Stud\ 89th Congress 1st Session House Document 124 Washmgton DC WhitesIde R E rt u/ (1973) Changes m Legal Vehicle Weights and Dlmenslons Some Economic EffeLts on Hlgh&dF\ NCHRP Report No 141 W‘i\hmgton DC

on Texas hlghways

431

Wmfrey R (1968) Econonm~ of the h~uumuun Lanus oj hforor 1 eh~cle Lkmenson~ und Weqhtc 1 vols FHWA-RD-73-69-70 Federal Hlghway Admmlstratlon, Washmgton DC Wmfrey R, Howell P D and Kent R M (1976) Truck Traffic Volume and Weight Ddtd for 1971 and Their Evaluations Report FHWA-RD-76-138 Federal Hlghway Admmlstratlon Washmgton D C