National Academies Press: OpenBook

Development of Clear Recovery Area Guidelines (2024)

Chapter: Chapter 3 - Vehicle Dynamics Encroachment Simulations

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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 3 - Vehicle Dynamics Encroachment Simulations." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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8 C H A P T E R   3 This chapter presents details of the vehicle dynamics encroachment simulations performed under NCHRP Project 17-11(03). Discussion on the selection of appropriate simulation code is presented next. This is followed by details of the vehicle models used in the encroachment simu- lations. The chapter also includes details of the simulation interface manager (SIM) program used for managing the large number of simulations performed, the various outputs recording for the simulations, and the stopping conditions used for managing the simulations in a batch mode. Details of the various driver inputs used in the simulations are also presented in this chapter. Simulation Code Selection The research team selected the multi-rigid-body vehicle dynamics analysis method for per- forming the large number of simulations needed in this project. CarSim, which is a commercially available vehicle dynamics code, was used for all the simulations performed. There were several considerations that weighted in favor of selecting the vehicle dynamics simulation method using CarSim as the analysis tool. These are discussed next. Computational Time Multi-rigid-body vehicle dynamics simulations have very short computational time require- ments. A 5-second encroachment event can usually be simulated in 3 to 5 seconds on a standard desktop computer. In contrast, simulation analysis methods, such as finite element analysis, can take more than 24 hours to complete the analysis of a similar event, using multiple CPU cores on a supercomputing machine. Since the research team anticipated performing more than a million vehicle encroachment simulations, the vehicle dynamics analysis method was selected for the simulation analysis. Accuracy Vehicle dynamics analysis makes simplifying assumptions about the vehicle and uses mostly lumped masses, springs, dampers, and nodal constraints to reasonably capture the dynamic response of the vehicle. While other analysis methods, such as the finite element method, can provide a lot more information about the stresses, strains, deformations, loads, etc. in different parts of the vehicle, that type of information is not needed to meet the objectives of this project. The simulation analysis was to be used to determine the overall trajectory, kinematics (i.e., vehicle roll, pitch, and yaw angles), and motion time histories (i.e., vehicle accelerations, speeds, etc.). For these, vehicle dynamics codes are known to provide reasonable and comparable results to the more advanced finite element simulation analysis (17). Vehicle Dynamics Encroachment Simulations

Vehicle Dynamics Encroachment Simulations 9   Vehicle Modeling and Design Characteristics CarSim, the vehicle dynamics code used for this project, provides many pre-built vehicle models in various vehicle classes. These pre-built vehicle models can be modified with relative ease using the basic vehicle geometric data and vehicle inertia and mass distribution. Reasonably accurate suspension properties can also be incorporated as needed. Since this project was going to develop more than one vehicle model, the ability to use pre-built models and customize them to specific vehicle makes and models was a significant advantage for using vehicle dynamics analysis with CarSim. Terrain Modeling CarSim also allows constructing roadway and roadside terrains with relative ease. This feature enabled the researchers to develop the several hundred roadway and ditch terrains that were analyzed in the project. Vehicle-Body-to-Terrain Contact Most vehicle dynamics codes are limited in the capability to simulate contact between two bodies. For vehicles encroaching onto roadsides with various steep foreslope and backslope com- binations, vehicle body-to-terrain contact can significantly influence the trajectory and overall stability of the vehicle. CarSim does not have a vehicle body-to-terrain contact by default. However, Texas A&M Transportation Institute (TTI) researchers previously developed a user subroutine for the com- mercial CarSim package that incorporates the vehicle body-to-terrain contact. This contact has been successfully used in several past research studies (18, 19). The contact subroutine checks if the vehicle’s body contacts the terrain during the simulation run time. If contact is detected, the subroutine applies contract forces to the vehicle to account for the terrain reaction. The contact subroutine tracks several user-defined points on the body of the vehicle and determines whether any of those points have penetrated the local terrain. If penetration is detected for a specific point, corrective forces are applied to the vehicle. Incorporating the vehicle body-to-terrain contact significantly improves the reliability of the simulation results in off-road conditions such as the roadside ditches modeled under this project. Soil-Furrowing Forces The default method in CarSim and most other vehicle dynamics analysis codes for applying road forces on vehicle tires is adequate for roadway surface conditions under normal vehicle turning and maneuvering on paved surfaces. In case of vehicle encroaching on roadside ditches, soil-furrowing forces that apply lateral force on the vehicle’s tires as it sideslips on wet soils are a significant source of vehicle rollover due to soil tripping. In previous studies, TTI researchers devel- oped a surrogate method to successfully incorporate soil-furrowing forces into vehicle dynamics simulations using a friction ellipse model (18, 19, 20). This surrogate method was implemented as a user subroutine that interacts with CarSim during each simulation runtime. The subroutine determines whether the vehicle is traversing a terrain marked as soil, and if so, calculates and applies lateral forces to the tire using the friction ellipse method. The effective lateral friction coefficient, msoil, is determined using the formulation shown in Figure 1. The lateral friction coefficient is higher for a vehicle sideslipping on soil than the default lateral friction coefficient on a paved roadway. The higher friction coefficient results in higher lateral force on the tires, which acts as a surrogate for the soil-furrowing force. The lateral friction coefficient is a

10 Development of Clear Recovery Area Guidelines function of the tire’s sideslip angle and is the same as the default roadway coefficient when there is no sideslipping. However, it increases with sideslipping and reaches a maximum value of mlimit when the vehicle is sideslipping at 90 degrees. Based on past research, the researchers used the mlimit value of 2.0 for the simulations (18,19). The researchers coded the ability to incorporate soil-furrowing forces only when the vehicle is traversing a terrain marked by the user as soil. This implies that when the vehicle is on a paved road or shoulder, the default CarSim friction formulation is used. The soil-furrowing forces are used only on the ditch terrain after the shoulder, which is more realistic. Driver Input Capability Another significant advantage of using CarSim vehicle dynamics simulations is the ability to apply realistic driver inputs to the vehicles during the simulations. Steering and braking inputs can be applied in a consistent manner for the simulations. Driver inputs used in this research are described in detail later in this chapter. Due to the many advantages described above, the vehicle dynamics simulation method using CarSim was a good choice for the simulation tool for this research project. Vehicle Model Selection The vehicle models used in the dynamic encroachment simulations were selected as repre- sentative of four categories: passenger car, pickup truck, compact utility vehicle (CUV), and sport utility vehicle (SUV). These categories were developed based on the statistical analysis of the 2019 U.S. vehicle sales data. Details on the classification of the 2019 vehicle sales data are presented in Chapter 4. The selected vehicles were modeled in CarSim and were used to perform the simulations. The passenger sedan category was represented by the 2019 Toyota Camry, which was the best-selling model in this category for 2019. Its average weight of 3,407 pounds also happens to be in the acceptable range for the Manual for Assessing Safety Hardware (MASH) 1500A midsize passenger vehicle (21). The pickup truck category was represented by the 2019 Ram 1500 Crew Cab 4-Door. While Ram pickups were collectively the second best-selling model in this category for 2019, a specific Figure 1. Friction ellipse model for modeling tire forces due to soil furrowing.

Vehicle Dynamics Encroachment Simulations 11   breakdown of sales for the half-ton class was not available. Preference was thus given to the use of the Ram 1500 as it is also the most used design vehicle for testing and evaluating crashworthi- ness of roadside safety hardware. The CUV category was represented by the Toyota RAV4, which was the best-selling model in this category for 2019. The SUV category was represented by the Jeep Grand Cherokee, which was the best-selling model in this category for 2019. Vehicle Model Development The research team developed vehicle dynamics models of the selected vehicle makes and models. The specifics of the selected vehicles and some of their key properties are presented in Table 2. The research team’s vehicle model development process is explained next. The process of modeling a vehicle model involved an evaluation of the “preset” vehicle models included with the CarSim software. The preset vehicle models in CarSim cover a wide range of vehicle designs and classes. For each of the selected vehicle makes and models, the researchers compared the vehicle properties of the selected model to those of the various preset CarSim models. Based on this comparison, the closest CarSim vehicle model was selected as a baseline, to which changes were made to match the specific vehicle of interest more closely. In addition to the vehicle properties listed in Table 2, other properties that were incorpo- rated into the vehicle models were the front and rear suspension types; steering type; antilock braking system (ABS); horizontal and lateral location of the vehicle’s center of gravity (CG); distribution of the vehicle’s curb mass between the front and rear axle; overall height, width, and ground clearance; front and rear overhang distances; and the yaw, pitch, and roll moments of inertia. Vehicle properties were obtained using the Expert AutoStats software database, which con- tains data for cars, pickups, vans, and utility vehicles (22). The Expert AutoStats software data- base is commonly used in the accident reconstruction field. Most of the design parameter values in the database are measured values, except the roll, pitch, and yaw moments of inertia. The values reported for moments of inertia are approximate and based on analytical calculations. In addition to incorporating design characteristics and properties of the vehicles, the researchers added coordinates of six hardpoints underneath the vehicle. These hardpoints are approximate locations of relatively stiff structural points underneath the vehicle that may engage the roadside terrain and apply terrain contact forces to the vehicle. TTI’s SIM program tracks these hardpoints Vehicle Year/Make/Model Representing Vehicle Class Curb Weight (lb) Wheelbase (in.) Track Width (front) (in.) Track Width (rear) (in.) CG Height (in.) 2019 Toyota Camry (L4) 4-Door Sedan Passenger sedan 3,492 111 62 63 22.37 2019 Ram 1500 (Classic) Crew Cab 4-Door 4x2 Pickup truck 5,360 149 68 68 29.61 2019 Toyota RAV4 4-Door 4x2 Compact utility (CUV) 3,492 106 63 64 26.73 2019 Jeep Grand Cherokee 4-Door 4x2 Sport utility (SUV) 4,650 115 64 64 27.13 CG = center of gravity. Table 2. Vehicle makes and models selected for simulation model development.

12 Development of Clear Recovery Area Guidelines after each timestep during a CarSim simulation and determines whether any of them contacted the terrain surface. If contact is detected, terrain forces are computed and applied to the cor- responding hardpoint(s) on the vehicle. The general locations of these six hardpoints are listed below and shown in Figure 2. 1. Driver-side, bottom of the front bumper. 2. Passenger-side, bottom of the front bumper. 3. Driver-side, middle of the vehicle. 4. Passenger-side, middle of the vehicle. 5. Driver-side, bottom of the rear bumper. 6. Passenger-side, bottom of the rear bumper. Simulation Interface Manager The TTI research team used an internally developed SIM program that generates various input files for the CarSim solver, runs CarSim in automated batch mode to perform analysis of all selected simulation cases, and generates the needed outputs for further analysis for research. The use of SIM greatly facilitated performing simulations of more than two million cases analyzed under this project. SIM performs the following functions. 1. Generates the needed CarSim input files, which include road/terrain profiles and events files (which include information about the vehicle’s encroachment speed, angle, and sideslipping and the driver’s steering, braking, and throttle information, etc.). 2. Runs CarSim in loop to perform analysis for all simulation cases in a defined simulation matrix. In doing so, the SIM program checks for vehicle body-to-terrain penetrations using TTI’s contact algorithm and applies terrain contact forces to the vehicle as needed. 3. Applies soil-furrowing forces to the vehicle tire if it determines that the tire is sideslipping while traversing on a wet soil terrain defined by the user. 4. Manages each simulation run time and terminates the simulations based on various termination criteria, such as if the vehicle returns to the road, travels too far, overturns, etc. 5. Generates output logs for all simulation cases, recording key simulation outcomes for further use in data analysis of the simulation outcomes. Further details of the SIM program can be found in previous work by Sheikh et al. (18, 19). Simulation Stopping Conditions Using SIM, the researchers set several conditions for determining whether a simulation should be stopped after the outcome of an encroachment case has been determined. This prevented the simulations from running longer than needed and saved time when a large number of Figure 2. Approximate locations of hardpoints for vehicle body-to- terrain contact (denoted by diamond shapes).

Vehicle Dynamics Encroachment Simulations 13   simulations needed to be performed. A simulation was stopped if any of the following conditions were met. 1. Vehicle’s CG came back to its initial lateral position, indicating that the vehicle had returned to the roadway. 2. Vehicle traveled beyond a specified lateral offset (set at 105 ft from the roadside edge of the travel lane). 3. Vehicle’s speed reduced below a specified minimum (set at 5 mph). 4. Vehicle rolled more than a specified maximum roll (set at 65 degrees). The vehicle was considered to have initiated an overturn at this point. 5. Vehicle pitched more than a specified maximum pitch (set at 90 degrees). The vehicle was considered to have overturned at this point. 6. Vehicle traveled for more than 10 seconds without any other significant outcome occurring. Simulation Outputs Even though CarSim generates outputs of individual simulations that include time histories of a large number of vehicle and terrain variables, to analyze more than two million simulation outcomes, there was a need to generate a single aggregated output of key variables from all simu- lations. Such aggregated output was to be used for statistical analysis in subsequent tasks of the research project. This was made possible using the output module of the SIM program, which generates an aggre- gated output table containing only selected output parameters from each simulation. Table 3 lists the aggregated outputs recorded by SIM. Additionally, vehicle trajectories of individual simulation cases were saved for further analysis. Label Description Run No. Simulation case number. Unique for a single-vehicle type only. Termination Describes if the simulation terminated normally or if the simulation crashed. It has values of “Normal” or “ERROR.” Outcome Stopping condition that caused the run to stop has the following values. - Time Exceeded - Returns - Stops - Gone Far - Overturns Description A brief description of the outcome. High Roll Flag for high roll (> 55 deg.). It has a value of 1 or 0 (1 = high roll). Max Roll Maximum vehicle roll during simulation (deg.). High Pitch Flag for high pitch (> 55 deg.). It has a value of 1 or 0 (1 = high pitch). Max Pitch Maximum vehicle pitch during simulation (deg.). Sideslip Flag for sideslipped vehicle (> 20 deg.). It has a value of 1 or 0 (1 = vehicle sideslip). Max Slip Maximum sideslip angle during simulation (deg.). Spinout Flag for vehicle spinout. It has a value of 1 or 0 (1 = vehicle spins out). Max Lat. Vel Maximum lateral vehicle velocity during simulation (km/h). Max Lat. Travel Maximum distance vehicle travels laterally from the edge of the roadway (m). Xcg at sim. Stop X-coordinate of vehicle’s sprung mass CG when simulation stops (m). Ycg at sim. Stop Y-coordinate of vehicle’s sprung mass CG from origin when simulation stops (m). Lateral Offset at sim. Stop Lateral distance of vehicle’s sprung mass CG from the edge of travel lane when simulation stops (m). Table 3. Simulation outcomes recorded in the aggregate outcomes table.

14 Development of Clear Recovery Area Guidelines Non-Tracking Encroachments When a vehicle leaves the roadway in a non-tracking manner, there is very limited crash data available to fully characterize the vehicle’s kinematic behavior. A vehicle is in a non-tracking condition when the heading vector is substantially different than the path of the vehicle’s CG. In several past research studies, non-tracking encroachments have been modeled by applying an initial yaw rate to the vehicle at the time of encroachment (e.g., 15 degrees/sec) as shown in the left illustration of Figure 3 (18, 19, 20). Yaw rate is the rotational velocity of the vehicle as it rotates about its vertical axis. In this scenario, the vehicle’s heading vector H and the path of the CG defined by the encroachment velocity V are initially aligned. A yaw rate (R) is applied at the start of the simulation, which initiates a sideslip mode resulting in a non-tracking encroachment. While this approach of modeling non-tracking encroachments is reasonable, the researchers felt that it could be improved and made more realistic by adding sideslip angle β to the non- tracking vehicle from the onset of the simulation as shown in the right illustration of Figure 3. This would be more realistic because in a non-tracking encroachment, a vehicle is expected to already be sideslipping as it leaves the travelway. Analysis of the NCHRP Web-Only Document 341 crash database was used to select a sideslip angle of 21 degrees for modeling the non-tracking encroachments (details in the next chapter) (3). As shown in the right side of Figure 3, the vehicle is now initialized with a heading vector, H, oriented away from the vehicle’s CG velocity vector, V, by the sideslip angle β of 21 degrees. The yaw rate, R, is still applied to the vehicle because it is expected that the vehicle will have some rotational velocity about its vertical axis that initiated the sideslip condition. Figure 3. Non-tracking encroachment models without sideslip (left) and with sideslip (right).

Vehicle Dynamics Encroachment Simulations 15   With the introduction of the sideslip angle to the non-tracking encroachment condition, the researchers felt it would be appropriate to conduct a small sensitivity study to assess if the 15-degrees/sec yaw rate from previous studies would still be appropriate. Several values of the yaw rate were simulated in the sensitivity study. The goal was to check that the 15-degrees/sec yaw rate, in combination with the sideslip angle, does not unrealistically destabilize the vehicle. The simulation matrix for assessing the sensitivity to yaw rate in non-tracking vehicle encroach- ments is presented in Table 4. A symmetric V-ditch with 1V:6H foreslope and backslope was selected as the terrain for the sensitivity study. Simulations were performed for three encroach- ment angles and three encroachment speeds (nine combinations). Four yaw rates were simulated for each of the nine encroachment speed and angle combinations. Figure 4 shows the maximum Slip Angle (degrees) 21 Yaw Rate (degrees/s) 0, 15, 30, and 45 Speed (mph) 45, 55, and 65 Angle (degrees) 10, 20, and 30 Foreslope 1V:6H Foreslope Width (ft) 30 Backslope 1V:6H Backslope Width (ft) 30 Ditch Bottom Width (ft) 0 Vehicle Type SUV Shoulder Width (ft) 6 Table 4. Matrix for yaw rate sensitivity study. Yaw Rate (degrees/s) Maximum Lateral Movement of Vehicle's CG Figure 4. Influence of yaw rate on the vehicle’s maximum lateral movement.

16 Development of Clear Recovery Area Guidelines lateral movement of the encroaching vehicles for the different yaw rates. Figure 5 presents the number of rollovers associated with each of the simulated yaw rates. Even though the number of cases simulated was small, the results indicated that the previously used yaw rate of 15 degrees/sec is still a reasonable choice. It did not excessively destabilize the vehicle, which is indicated by the number of rollovers remaining lower than the other yaw rates simulated (see Figure 5). The results also suggested that the non-tracking encroachments are not highly sensitive to the yaw rate. This can be concluded by noting that doubling the yaw rate from 15 degrees/sec to 30 degrees/sec resulted in only one additional rollover. Furthermore, the extent of lateral movement distribution remained the same for these two yaw rates as shown in Figure 4. Based on these observations, the researchers selected a 15-degrees/sec yaw rate combined with a sideslip angle of 21 degrees for simulating the non-tracking encroachments in the simulation matrix. Driver Inputs Response of the driver at the time of and during the roadside encroachment is a complex phe- nomenon with many possible variations in driver response. The research team developed five driver inputs for simulating a broad spectrum of driver responses that can be expected. Since driver response at the time of the encroachment is expected to depend on whether the vehicle is tracking or non-tracking, the research team incorporated the tracking/non-tracking vehicle condition as part of the driver inputs. The five driver inputs used in this research are presented in Table 5. Driver Input 1 simulated a driver who is asleep or impaired and does not apply any driver input. Inputs 2, 3, and 4 were tracking encroachments in which the driver reacts after a perception-reaction time (PRT) of 1 second. The 1-second PRT has been used in previous research and was considered reasonable for this research (18, 19). Figure 5. Influence of yaw rate on number of vehicle rollovers.

Vehicle Dynamics Encroachment Simulations 17   Input 2 depicts a panic steer back to the roadway without brakes being applied. Input 3 depicts a panic braking response without any steering input. Input 4 depicts both, a steer back to the roadway and braking. Input 5 was a non-tracking encroachment. In this case, the driver was assumed to have already reacted to some event on the roadway and had applied the steering and/or braking inputs prior to encroaching. Thus, no PRT was used for this input at the start of the encroachment. For all the tracking steering inputs, the rate for panic steer was determined based on NHTSA’s Fishhook maneuver guidelines, which have a recommended steering rate of 720 degrees/second. This rate was used to develop a maximum steer of 360 degrees after the passage of PRT. Simulation Matrix Table 6 presents the vehicle dynamics encroachment simulation matrix used in this research. The variables in the matrix include vehicle type, encroachment speed and angle, vehicle orien- tation at departure (i.e., tracking or non-tracking), driver input (e.g., steering and/or braking), Driver Input Details 1 No input (tracking). 2 Panic steer to return to road (R2R), no braking (tracking). After 1.0 sec PRT delay on leaving the edge of the travel lane, a 360-deg steer toward the roadway is applied at the rate of 720 deg/s. 3 Full braking, no steering (tracking). After 1.0 sec PRT delay on leaving the edge of the travel lane, sudden brakes are applied. Brakes are modeled as ABS brakes. 4 Panic R2R steer and full braking (tracking). After 1.0 sec PRT delay on leaving the edge of the travel lane, sudden brakes and a 360-deg steer toward the roadway are applied at the rate of 720 deg/s. Brakes are modeled as ABS brakes. 5 Constant steer and full ABS brake (non-tracking). Vehicle encroaches with a yaw rate of 15 deg/s (yawing toward the roadway) and a sideslip angle of 21 degrees, with a constant steer angle of 360 deg and fully applied ABS brakes. Table 5. Driver inputs for the encroachment simulations. Table 6. Vehicle dynamics encroachment simulation matrix. Variable Values Vehicle Type (also see Table 2) Passenger sedan Pickup truck Compact utility Sport utility Encroachment Speed 35, 45, 55, 65, and 75 mi/h. Encroachment Angle 5, 10, 15, and 25 degrees. Driver Input and Vehicle Orientation (also see Table 5) 1 – No input – tracking 2 – Steering only after PRT – tracking 3 – Braking only after PRT – tracking 4 – Steering and braking after PRT– tracking 5 – Fixed steer and full brakes without PRT – non-tracking Shoulder Width (ft) 2, 6, and 12. Shoulder slope of 4% on all shoulders. Foreslope Ratio 1V:10H, 1V:6H, 1V:4H, and 1V:3H. Foreslope Width (ft) 8 and 16. Ditch Bottom Width (ft) 0, 4, and 10. Backslope Ratio 1V:6H, 1V:4H, 1V:3H, and 1V:2H. Backslope Width (ft) 8 and 16. Horizontal Curvature (degrees) 0, 4, and 6. Superelevation of 2% on all curves. Vertical Grade (percent) 0, 4, and 6. All downgrade.

18 Development of Clear Recovery Area Guidelines horizontal curvature, vertical grade, shoulder width, foreslope ratio, foreslope width, ditch bottom width, backslope ratio, and backslope width. The roadway and roadside design parameters were selected to represent a range of typical roadside conditions. Even though superelevation and shoulder slope were not selected to be part of the final clear zone guidelines, the researchers incorporated typical mild values for these slopes so that the sim- ulated roadway configurations were more representative of the field conditions. The researchers examined various state standards and selected a shoulder slope of 1:25 and a superelevation of 2% for the road profile modeled in this research. Other than the horizontal curves, the simulated roadway profile was flat and level. The selected simulation matrix consisted of 2,073,600 individual simulation cases. The researchers used SIM and CarSim to perform all the simulations. The output generated from the simulations was further used in the statistical analysis and clear zone guideline development, as presented in the subsequent chapters of this report.

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The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidance that evolved over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors.

NCHRP Research Report 1097: Development of Clear Recovery Area Guidelines, from TRB's National Cooperative Highway Research Program, develops updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. These updated guidelines can aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.

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