Accident Involvement Rates for Trucks

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Nmero 29, 2007 Engenharia Civil UM 5

Revising the AASHTO Curve: Accident Involvement Rates for Trucks andSpeed Differentials on Highway Grades

Ricardo A. de Melo 1,

Federal University of Paraba, Centre of Technology, Department of Civil Engineering

Joo Pessoa, Paraba, Brasil

Jos Reynaldo Setti2

University of So Paulo, Engineering School of S. Carlos,

Department of Transportation EngineeringSo Carlos, So Paulo, Brasil

ABSTRACT

This paper presents as suggestion the adaptation of the AASHTO criteria for the design ofclimbing lanes on Brazilian highways, which focus on the development of relations betweenaccident involvements rates of trucks and speed distribution of vehicles. Two relations are

proposed for Brazil: one for two-lane highways and one for divided multi-lane highways. The proposed relations were developed using accident data from specific upgrades to ensure that onlyaccidents involving slow-moving trucks that could be prevented by a climbing lane were takeninto account. The speed distributions of vehicles were surveyed on upgrades, to allow for a morerealistic estimate of the speed differential both for trucks and faster moving vehicles. Themaximum allowable speed reductions for Brazilian trucks on upgrades (20 km/h, for two-lanehighways, and 30 km/h for divided multilane highways) were obtained from the crash

involvement rate associated with the 15 km/h speed differential defined by AASHTO guidelines .The decision to modify the speed difference in the current geometric design standards, however,must be supported by additional data to be obtained with further research, which should be baseda larger and more representative database than the one used in this study. It is also imperative todetermine a crash involvement rate which would represent an acceptable compromise betweensafety and capital expenditures for the construction of climbing lanes, as the one adopted in thisstudy was arbitrarily chosen to demonstrate the application of the method.

1 Professor Adjunto [email protected] Professor Adjunto


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6 Engenharia Civil UM Nmero 29, 2007

1. INTRODUCTION

The widespread use of the AASHTO Green Book (AASHTO, 2001) around the worldmakes it the most important reference for the design of climbing lanes. As such, guidelines usedoutside the U.S. (MTO, 1990; ATU, 1995; Mendonza and Mayoral, 1994; Schulze and Lamm,2000) are usually derived from AASHTO, including the current design guidelines for federalhighways in Brazil (DNER, 1999). The Brazilian standards for rural highway design areestablished by the National Department for Land Transportation Infrastructure (DNIT), throughthe Highway Research Institute (IPR), which published the first instructions for climbing lanedesign in 1979 (DNER, 1979). These guidelines were replaced in 1999, by the Rural HighwayDesign Manual (DNER, 1999), which also includes standards for the design of passing lanes,

passing bays and three-lane highways. However, the climbing lanes criteria are a simplytranslation of the AASHTO guidelines, without any adaptation for Brazilian conditions. Inaddition, the AASHTO criterion about relation between crash involvement rates for trucks andspeed differentials need revision. The following aspects must be considered, if the AASHTOclimbing lanes criteria are to be adapted to different contexts than the U.S.: ( i) speed-distancecurves for typical heavy trucks on upgrades; ( ii) maximum speed reduction for trucks onupgrades; and ( iii) upgrade traffic flow rate and upgrade truck flow rate. The aim of this paperwas to create a new relation between crash involvement rates for trucks and speed differentials, todeterminate the maximum speed reduction for trucks on upgrades and to justify climbing lane ontwo-lane and on divided multilane rural highways in Brazil.

2. LITERATURE REVIEW

Excessive speed difference between two vehicles on highway grades can potentially leadto severe accidents. Several studies have shown that crash involvement rates are related withvehicles speed distribution. An widely cited study (Solomon, 1964), that compared the estimatedspeed of vehicles involved in accidents with the measured speed of control vehicles, using datafrom 35 highway segments in 11 states in the U.S. (75% of them on two-lane roads), concludedthat vehicles traveling at approximately 10 km/h faster than the average traffic speed had thelowest crash involvement rate (Aarts and Schagen, 2006). This study also concluded that vehiclestraveling at speeds much lower or much higher than the average had greater accident involvementrates, as shown in Figure 1. These results were later confirmed by two other studies (Cirillo,1968; USDT, 1970). While more recent studies (Aarts and Schagen, 2006; Kloeden et al., 2001)have criticized the U-shape curve proposed by these older reports (Solomon, 1964; Cirillo, 1968;USDT, 1970), pointing out that the crash involvement rates for vehicles traveling at speeds below

the average have been overestimated, the literature seems to agree that larger differences inspeed between vehicles are related to a higher crash rate (Aarts and Schagen, 2006).

Climbing lanes, which remove slow moving vehicles from the traffic stream, are aneffective way of increasing the safety of highways with low expenditure. In fact, the AASHTOguidelines establish that safety considerations may justify the addition of a climbing lane,regardless of grade and traffic volumes (AASHTO, 2001). Several studies (Harwood et al., 1985;Hauer and Persaud, 1996; Jain and Taylor, 1991) have shown that climbing lanes can effectivelyreduce the number and severity of accidents on upgrades, since slower vehicles are diverted tothe climbing lane, freeing the traffic lane for the faster ones.


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Figure 1 - Relationship between crash involvement rate and variation from average speed(adapted from Solomon, 1964)

According to the metric version of the Green Book (AASHTO, 2001), the construction ofa climbing lane on a two-lane highway is warranted if the following criteria, which reflecteconomic considerations, are satisfied:

1. Traffic flow rate on the upgrade exceeds 200 vehicles/hour; and

2. Truck flow rate on the upgrade exceeds 20 trucks/hour; and3. And one of the following conditions should be satisfied:

a. Trucks with a mass-to-power ratio of 120 kg/kW experience a reduction of 15 km/hor greater on the upgrade; or

b. The level of service for the upgrade is E or F; orc. A reduction of two or more levels of service is verified when moving from the

approach segment to the upgrade.The speed reduction equal to 15 km/h defines the critical length of grade, which it is the

length of a particular rising grade that reduces the speed of trucks about 15 km/h below theaverage running speed of the remaining traffic (AASHTO, 2001). The critical length of grade isrelated to traffic safety and reflects the maximum speed dispersion that should be desired on an

upgrade, considering that greater dispersion could potentially lead to more accidents.The Green Book points out that climbing lanes are less common in multilane highways

because the additional lane (or lanes) makes the passing maneuver easier and provides enoughcapacity to handle the traffic demands and the typical fraction of low performance trucks. Besidesthe increased risk of accidents, the aspects to justify climbing lanes on multilane highways are(AASHTO, 2001): ( i) critical lengths of upgrade, ( ii) effects of trucks on the traffic flow ongrades in terms of equivalent passenger-car flow rates; and ( iii) service flow rates for the desiredlevel of service and the next inferior level of service. Economic considerations are also involvedin determining the need for climbing lanes in multilane highways. The Green Book suggests thatclimbing lanes are not generally justified on four-lane highways with directional flow rate below1000 vehicles/(hour.lane), regardless of the percentage of trucks.

-40 -30 -20 -10 0 +10 +20 +30VARIATION FROM AVERAGE SPEED, M.P.H.

100,000

50,000

10,000

5,000

1,000

500

100

I N V O L V E M E N T R A T E

NIGHT TIME

DAY TIME


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The 15 km/h maximum speed reduction criterion used to establish the critical length ofgrade was adopted based on a study by Glennon (1970), whose objective was to find the speedreduction that would justify the construction of a climbing lane. However, this study did not usedata from any specific grade or unsafe location to establish the 15 km/h speed reduction criterion.Speed distributions were obtained from sensors located on level segments and crash involvementrates for trucks were derived from the average accident rates obtained by Solomon (1964). Someof the study assumptions can be questioned, such as the one that states that the average speed ofthe traffic stream on a rising grade is equal to the average speed observed on level segmentsminus 30% of the speed reduction experienced by the design truck on that grade. Another pointthat raises concerns is the lack of information about the types of accidents used to calculate thecrash involvement rate, considering that there are various accident types that are not related toslow moving trucks. Despite the studys shortcomings, it should be stressed that Glennon was thefirst to use a systematic and rational approach to establish the maximum allowable speedreduction that warrants the construction of a climbing lane, which produced the curve shown inFigure 2. As a conclusion, he suggested that 25 km/h speed reduction should be changed to 15km/h, a suggestion that was accepted by AASHTO. The speed reduction curve shown in Figure 2has been repeated in every revision of its design policies, since then. A justification for thecontinued use of this curve is that, should this value be wrong, it would be wrong in aconservative way, increasing safety rather than reducing it.

0 5 10 15 20SPEED REDUCTION (mph)

4000

3500

3000

2500

2000

1500

1000

500

0

I N V O L V E M E N T R A T E

Figure 2 - Crash involvement rate for trucks vs. speed reduction relationship (adapted from

Glennon, 1970)

In Germany (Schulze and Lamm, 2000), the criteria adopted also consider the speedreduction due to the road bendiness (measured in degrees/km). The speed reduction experienced

by the design truck (130 Kg/KW) on the upgrade is then added to the speed reduction due tohorizontal curves, which is experienced both by cars and trucks. A climbing lane is justified if the15 th-percentile speed for trucks is less than half of the 85 th-percentile speed for cars.

Another example of adaptation of AASHTO guidelines was developed in Alberta,Canada. The speed reduction criterion uses two performance curves, one for a 180 Kg/KWvehicle the design truck and another for a 120 Kg/KW vehicle, to be used when buses and


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recreation vehicles are 85% or more of the heavy vehicle traffic (ATU, 1995). The minimumflow rate criterion is established in terms of percent of passing zones, grade magnitude and lengthand percent trucks in traffic. This criterion is associated with two other conditions: average heavyvehicle daily traffic greater than 150 and service level at the peak hour lower than A.

Among the previous studies in Brazil, one (Kabbach, 1992) has attempted for changes inthe AASHTO guidelines to Brazilian conditions. The study suggests a loaded design truck andflow rates to justify the construction of climbing lanes on upgrades of Brazilian two-lanehighways.

3. METHOD

The purpose of the research reported in this paper was to develop a procedure to adapt theAASHTO criterion for climbing lanes, within a Brazilian context: ( i) speed-distance curves fortypical heavy trucks on upgrades; ( ii) maximum speed reduction on upgrades; and ( iii) estimationof traffic flow rates that justify the construction of a climbing lane. This paper was focused on thesecond aspect, the development of a relation between accident involvement rates for trucks andspeed difference between heavy vehicles and faster vehicles that could be used to replace thecurve used by AASHTO, shown in Figure 2. The other aspects of the proposed approach arereported in other papers (Demarchi et al. 2003; Melo and Setti, 2003).

In planning the development of the proposed procedure, some care was taken to avoid theshortcomings of Glennons approach. The steps in the proposed method comprised: ( i) survey ofdata on accidents, flow rates and speed on upgrades at sites with greater occurrence of accidentsthat involve slow moving and faster moving vehicles; ( ii) estimation of average daily traffic andspeed distributions; ( iii) estimation of accident involvement rates for trucks; and ( iv) estimationof the relationship between accident involvement rates and speed differential to obtain themaximum allowable reduction in speed on upgrades, for two-lane and divided multilanehighways. The use of accident data from specific sites ensured that only accidents involvingslow-moving trucks and that could be prevented by a climbing lane were taken into account; thespeed data collected on the upgrades, for trucks and faster moving vehicles, allowed for a morerealistic estimate of the speed distribution.

The adopted approach used relationships between accident involvement rates for trucksand the speed differential for cars and trucks in the traffic stream on the upgrade, rather than thedifference obtained from estimated speeds of individual vehicles involved in accidents. Thisapproach was adopted to eliminate a potential bias that could be introduced by incorrect speedestimates.

3.1. Data collection

The highway network used for the data collection comprised 770 Km of typical roads inthe state of So Paulo, in Southeastern Brazil, divided into 480 km of two-lane roads and 290 kmof divided, multilane highways. Data were obtained from highway operators from the records ofall notified accidents on the network over a period from 34 to 35 months (1017 to 1047 days). Alltraffic count data for nearby counting locations were also obtained, as well as classified trafficdata from several toll plazas that exist on or close to the network. Toll plaza traffic data covered a

period of 24 months.From the accident database, only accidents involving at least one truck were used; among

these, only accidents that are typically related to slow-moving trucks (such as rear-end accidents,


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collision during passing maneuvers, etc.) were selected. The chosen sites are shown in Tables 1and 2.

In each of these locations, flow rate and traffic composition data were collected for atleast 4 hours, to complement traffic counts and disaggregate flow rates obtained from thesurrounding toll plazas. Based on these data, estimations of the annual average daily traffic for

both total traffic and truck traffic were made for each site. Traffic sensors installed in each trafficlane were used to collect data on individual vehicle speed and length; at the same time, trafficflows were also recorded on video to ensure a correct classification of the vehicles.

3.2. Estimation of Accident Involvement Rates for Trucks

Crash involvement rates for trucks were calculated using the ratio between the number oftrucks involved in accidents and a measure of truck exposure:

810= d AADT L NTI

IRT t

, (1)

with: IRT : crash involvement rate for trucks, in trucks involved in accidents that could be prevented by a climbing lane by 100 million truck.km;

NTI : number of trucks involved in accidents at this site; L: length of grade (km); AADT t : estimated annual average daily truck traffic; andd : period of analysis (days)

Table 1 - Crash involvement rates for trucks on two-lane highways (in 10 8 truck.km of travel)Site Total accidents Trucks involved AADT Truck AADT Grade length (km) Period of analysis (days) Crash involvement rate1 6 13 2471 1058 1.54 1017 7852 7 13 2155 765 2.32 1017 7203 (upgrade) 4 5 3079 924 1.30 1047 3983 (downgrade) 2 2 6084 2020 1.30 1047 734 2 4 2478 634 1.00 1047 6025 2 4 3120 640 1.40 1047 4266 4 5 1467 453 1.10 1047 958

Table 2 - Crash involvement rates for trucks on divided multilane highways (in 10 8 truck.km oftravel)Site Total accidents Trucks involved AADT Truck AADT Grade length (km) Period of analysis (days) Crash involvement rate

7 18 32 7825 3380 1.88 1017 495

8 1 3 8224 3939 0.50 1017 1499 19 30 8585 3588 3.10 1017 265

10 4 5 7666 2874 2.14 1017 8011 1 2 8998 3509 1.92 1017 2912 3 6 8993 2059 0.70 1047 39813 3 4 10535 2802 1.00 1047 13614 9 15 8342 2569 1.70 1047 32815 5 7 8615 2860 2.50 1047 94


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Tables 3 and 4 also show the estimates for crash involvement rate for trucks at each site inthe sample. For site 3, the downgrade rate was also estimated, to provide a data point with littledifference between the speeds of cars and trucks, as shown in Table 3.

Automatic traffic recorders, capable of registering each vehicles length and speed, wereinstalled on every lane at each site. The sensors were installed on the upgrade at a point wheretrucks travel on crawl speed, to allow for a more accurate estimation of the speed difference

between passenger cars and trucks. The data were used to obtain the speed distributions for thesevehicle classes; the mean speed and V 85 and V 15 were found for each site. The 85 th and 15 th

percentile speeds were used to represent the speeds of the fast-moving and slow-moving vehicleson the upgrade. The results are summarized in Tables 3 and 4, which also show the speeddifferentials found at each site.

4. PROPOSING A RELATION BETWEEN ACCIDENT INVOLVEMENT RATES ANDSPEED FOR BRAZILIAN HIGHWAYS

The estimate of the speed differential depends on the definition of the speeds of faster andslower vehicles, as Tables 3 and 4 illustrate. To determine what would be the best definition, fourcombinations among mean speed (for cars and trucks), V 85 for cars and V 15 for trucks were

plotted against estimates of crash involvement rates for trucks at each site. Using regressionanalysis, an exponential function was fit to each data set; the set with the best regressioncoefficient ( R2) was chosen as the representative relationship between speed differential andcrash involvement rates for trucks.

Table 3 - Speed distributions and speed differential estimates for two-lane highway sitesObserved speed distributions (km/h)

Cars Trucks Speed differential (km/h)Site VC mean VC85 VT15 VTmean VCmean VT15 VCmean VTmean VC85 VT15 VC85 VTmean 1 83.6 100.0 38.6 57.0 45.0 26.6 61.4 43.02 70.7 92.2 27.0 48.6 43.8 22.1 65.2 43.63 (upgrade) 82.6 97.0 39.0 63.7 43.6 18.9 58.0 33.33 (downgrade) 67.2 90.0 48.0 67.2 19.2 0.0 42.0 22.84 84.9 100.0 27.0 61.0 57.9 23.9 73.0 39.05 89.8 105.3 50.0 70.2 39.8 19.6 55.3 35.16 97.0 109.0 58.0 76.5 39.0 20.5 51.0 32.5

Table 4 - Speed distributions and speed differential estimates for divided, multilane highway sitesObserved speed distributions (km/h)Cars Trucks Speed differential (km/h)

Site VC mean VC85 VT15 VTmean VCmean VT15 VCmean VTmean VC85 VT15 VC85 VTmean 7 84.9 101.0 35.0 55.7 49.9 29.2 66.0 45.38 97.8 109.0 66.0 79.2 31.8 18.6 43.0 29.89 93.9 108.0 50.0 70.3 43.9 23.6 58.0 37.7

10 92.0 111.0 61.0 77.5 31.0 14.5 50.0 33.511 95.0 111.0 45.0 78.4 50.0 16.6 66.0 32.612 95.7 113.0 43.0 78.1 52.7 17.5 70.0 34.913 86.5 108.0 63.0 83.1 23.5 3.4 45.0 24.914 85.3 100.0 39.0 57.7 46.3 27.5 61.0 42.315 88.4 106.0 39.0 66.2 49.4 22.2 67.0 39.8


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The best relationship for two-lane highways was the one that estimates speed differentialusing the mean speeds for cars ( VC mean ) and trucks ( VT mean ):

[ ]77.540 exp 0.0941 ( ) ,mean mean

IRT VC VT = R2 = 0,8928 (2)with accident involvement rates given in number of trucks involved in accidents per 10 8 truck.kmand speeds in km/h. Figure 3 illustrates this relationship and shows a comparison with theAASHTO curve. The proposed curve suggests that a 20 km/h speed reduction would produce thesame crash involvement rate as the 15 km/h AASHTO guidelines.

0

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Speed differential (km/h)

C r a s

h i n v o

l v e m e n

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t e

( t r u c

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/ 1 0 0 m

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t r u c

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2 lanes (observed)

Multilane (observed)

AASHTO

2 lanes (model)

Multilane (model)

Figure 3 - Relationships between speed differential and crash involvement rates for trucks

For divided multilane highways, the best fit was also obtained using the difference between the mean speed for cars ( VC mean ) and the mean speed for trucks ( VT mean ). This function,however, has two segments ( R2 = 3.10 -8; R2 = 0,6777; respectively):

[ ]113.300, for ( ) 18.6 km/h

10.311 exp 0.1276 ( ) , otherwise,mean mean

mean mean

VC VT IRT

VC VT


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allow more freedom for maneuvers within the traffic stream. For same safety level of the 15 km/hspeed reduction proposed by AASHTO (2001), this study suggests 20 km/h and 30 km/h forspeed reduction criterion on upgrades of two-lane and divided multilane Brazilian highways,respectively, to justify the construction of climbing lanes.

5. CONCLUDING REMARKS

One of the criteria adopted by AASHTO to determine the need for a climbing lane is based on the speed reduction experienced by trucks on the rising grade. This speed reduction isused to determine the critical length of grade. Currently, AASHTO adopts a 15 km/h speedreduction for trucks to define the critical length. In the proposed adaptation AASHTOsguidelines for climbing lanes in a Brazilian context, the speed reduction was determined using therelationship between observed crash involvement rates for trucks and the observed difference inspeed between trucks and faster vehicles, for specific upgrades. The suggested maximum speedreduction allowable (20 km/h, for two-lane highways, and 30 km/h for divided multilanehighways) was obtained using the crash involvement rate associated with the 15 km/h speedreduction allowed by AASHTO guidelines. The proposed approach avoids some of the problemsassociated with Glennons approach, namely, the lack of accident and speed data from climbinggrades.

The decision to modify the speed difference in the current geometric design standards,however, must be supported by additional data to be obtained with further research, which should

be based a larger and more representative database than the one used in this study. It is alsoimperative to determine a crash involvement rate which would represent an acceptablecompromise between safety and capital expenditures for the construction of climbing lanes, asthe one adopted in this study was arbitrarily chosen to demonstrate the application of the method.

ACKNOWLEDGEMENTS

The authors acknowledge the support of Centrovias Sistemas Rodovirios S/A andTringulo do Sol Auto-Estradas S/A, who kindly provided a significant portion of the data usedin this study. Advice and suggestions from Professor John Morrall (University of Alberta,Canada) are also acknowledged. This research was supported partially by grants from CNPq,FINEP and FIPAI.

REFERENCES

Aarts, L. and I.v. Schagen. Driving Speed and the Risk of Road Crashes: a Review. Accident Analysis & Prevention . Vol. 38, pp. 215-224 (2006).

AASHTO. A Policy on Geometric Design of Highways and Streets . American Association ofState Highway and Transportation Officials. Chapter III: Elements of Design, p. 235-254. FourthEdition. Washington, D. C. (2001).

ATU. Highway Geometric Design Guide . Chapter B: Alignment Elements. AlbertaInfrastructure, Alberta Transportation and Utilities. p. B-55 to B-79. Alberta, Canad (1995).

Cirillo, J.A. Interstate System Crash Research: Study II, Interim Report II. Public Roads . Vol.35, No. 3, pp. 71-75 (1968).

Demarchi, S.H., R.A. Melo, and J.R. Setti. Validation of a Truck Performance Model.Transportes . Vol. 9, No. 1, 2003, pp. 53-68 (in Portuguese) (2003).


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DNER. Instrues para Implantao de Terceiras Faixas (Guidelines for Climbing LaneDesign). Ministrio dos Transportes, DNER, Rio de Janeiro, Brazil (1979).

DNER. Manual de Projeto Geomtrico de Rodovias Rurais (Rural Highways GeometricDesign Manual). Publication IPR-706. Ministrio dos Transportes, IPR-DNER, Rio de Janeiro (1999).

Glennon, J.C... An Evaluation of Design Criteria for Operating Trucks Safely on Grades. Highway Research Record , No. 312, pp. 93-112 (1970).

Harwood, D., A.D.S. John, and D.L. Warren. Operational and Safety Effectiveness of PassingLanes on Two-Lane Highways. Transportation Research Record : Journal of the TransportationResearch Board, No. 1026, pp. 31-39 (1985).

Hauer, E. and B. Persaud. Safety Analysis of Roadway Geometric and Ancillary Features .Transportation Association of Canada, Ottawa, Canada (1996).

Jain, M.K. and W.C. Taylor. Criteria for Passing Relief Lanes on Two-Lane Highways. ITE Journal . Vol. 61, No. 2, pp. 25-30 (1991).

Kabbach, F.I. Contribution to the Study of Climbing Lanes Design Guidelines on Two-Lane Highways . Ph.D. Thesis (in Portuguese), PTR-EP, University of So Paulo (1992).

Kloeden, C.N., A.J. McLean, V.M. Moore, and G. Ponte. Travelling Speed and the Risk ofCrash Involvement on Rural Roads RARU , Adelaide University, Australia (2001).

Melo, R.A. and J.R. Setti. Climbing Lanes for Brazilian Two-Lane Highways: an Adaptationof AASHTO Guidelines. Transportes . Vol. 9, No. 2, pp. 41-48 (in Portuguese) (2003).

Mendoza, A. and E. Mayoral. Economic Feasibility Assessment Procedure for Climbing Laneson Two-Lane Roads in Mexico. Transportation Research Record : Journal of the TransportationResearch Board, No. 1457, 1994, pp. 26-34 (1994).

MTO. Heavy Vehicle Performance on Grade and Climbing Lane Criteria . DocumentationPage TDS-90-11. Ministry of Transportation. Ontario (1990).

Schulze, C. and R. Lamm. A Procedure for Designing Auxiliary Lanes on Upgrade Sections ofTwo-Lane Rural Roads . In: Proceedings of the 2nd. International Symposium on Highway GeometricDesign. Mainz, Germany. TRB, pp. 472-485 (2000).

Solomon, D. Crashes on Main Rural Highways Related to Speed, Driver and Vehicle . Bureauof Public Roads, U.S. Department of Commerce, Washington, D.C. (1964).

USDT. Speed and Accidents . U.S. Department of Transportation, Washington, D.C. (1970).

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