Disclaimer: These recommendations are those of the authors, and do not necessarily reflect the positions of the NIOSH, the University of Michigan, or our other sponsors.
Context: Although considerable progress is being made in understanding ingress/egress behavior and risks, some basic facts regarding injuries associated with ingress/egress systems are still unknown. For example, we do not know whether slips or trips are a more important cause of injury events. We have good evidence that the risk of injury in egress is higher than in ingress, but we don't know when during the egress sequence the injury-producing event typically differs from an ordinary egress. We know some drivers slip when contacting the ground, but we don't know if those events are more common than slipping on the steps, or whether they are more or less likely to cause injury than slipping on the steps. We don't know the extent to which trips (foot catching on a step during outward-facing egress) are a factor in causing injury, although we suspect that trips are less important than slips.
We also have only limited information about how system configuration affects risk. Recent research has demonstrated that step and handhold configurations, along with driver factors, affect the forces on the driver's body. But we don't know whether those increased forces correspond to a meaningful increase in either acute or chronic injury. We also do not have a good way to quantify step slip-resistance in a manner that is relevant to truck steps, although we are working on that, starting with quantifying realistic foot force vectors.
Nonetheless, we believe that our current understanding of the problem is sufficient to make a variety of recommendations, sometimes based on more what to avoid than what is optimal. We welcome suggestions and alternative interpretations of the available data.
Assumptions on which these recommendations are based:
1. Ingress/egress system design for highway trucks should focus more on reducing the risk of acute injury (slips and falls) than on minimizing musculoskeletal injury risk due to repetitive loading.
2. Most injuries occur due to slips (rather than trips) on egress. Slipping on ingress is the next most important mechanism.
3. Most injury causing events are initiated by a foot slip, rather than a hand slip.
4. The primary function of handholds is to provide stability during ingress/egress, and, in unusual situations, to mitigate the effects of a foot slip.
5. Drivers will exit the truck facing outward up to half the time -- the system should be designed to accommodate that behavior as safely as possible.
The following sections present our recommendations in several areas:
Construction –– Steps should be constructed to provide good slip resistance and to shed contaminants well. Unfortunately, standardized procedures for assessing both of these that are applicable truck steps are not available. Based on our examination of steps in use on trucks, our preference is for grating-type steps, which provide both the highest slip resistance and best shed contaminants. If perforated metal steps are used, they should have large, well-located drain holes (some styles do not) to minimize the buildup of water, snow, and ice.
We believe a step should have a relatively small (sharp) radius on the outer, upper edge, rather than a rounded edge. Drivers interact with the edge routinely, particularly on ingress, and many steps have smooth edges that provide little slip resistance.
We recognize that a grating step with a sharp edge likely increases the risk of tripping for outward-facing egress. In this type of event, the driver's trailing foot snags on the step, potentially causing a fall or sprain. Good data on the relative prevalence of slip and trip events on egress are not available. But, based on our analysis of truck egress tactics, we believe that slips are both more common and more likely to produce injury when they occur. Hence, we believe that it makes sense to bias the step design toward reducing the risk of slips rather than compromising slip resistance for reduced trip risk.
The design of some truck steps appears to be influenced by the design of stair nosings. Stair design has received much more attention than truck step design and considerable standardization of stair designs has occurred. On stairs, trips due to sharp edges on the stair nose are a concern. However, caution must be exercised in applying stair design concepts to truck steps, because truck steps are used differently. The greater vertical spacing of truck steps means that drivers interact primarily with the edge of the step when the foot first contacts the step on ingress. On egress, drivers who face the truck (the preferred tactic) back down the steps, with foot motions that are substantially different from people walking forward down stairs. Even drivers who exit facing away from the truck usually face sideways to the steps (either toward the rear of the truck or toward the open door) and move their feet differently than people walking down stairs. Hence the risk and kinematic effects of tripping are different than on stairs.
A sharp step edge increases the risk of injury if a body part strikes the edge during a fall. Clearly the edge should not be knife-sharp -- it needs only to penetrate into the tread of the driver's footwear to great highly slip-resistant edge. Some innovation in edge design to maximize slip resistance for an angled shoe while minimizing the risk for fall contact would be valuable. In examining injury data, we see few injuries apparently related to contact with the step compared with a large number of injuries due to body motions during the fall itself (sprains, strains), or contact with the ground.
Summary: We recommend grating steps, well maintained, with a slip-resistant edge. Perforated metal steps with good drainage, prominent slip-resistance features, and a slip-resistant edge, would be expected to perform equivalently.
Step Layout --The goal of step layout is to arrange the two steps relative to the door opening to provide the safest entry and exit. The steps are part of a system, and so cannot be considered in isolation, but several guidelines for step placement are useful. Of course, the placement of steps is constrained by a number of factors, including the width of the cab and the location of other features, such as the fuel tanks.
Step placement, as part of the the ingress/egress system, may affect the tactics drivers choose. Conceptually, a ladder-like vertical arrangement of steps would be expected to induce a larger percentage of drivers to use an inward-facing egress tactic. Yet, such a step arrangement would not generally be considered safer because the consequences of a slip on either ingress or egress would be greater, and the overall difficulty of both ingress and egress would be much greater than with a more typical arrangement. Similarly, a stair-like arrangement with vertical step offsets similar to the horizontal offsets would probably not be desirable, even if space were available, because it would be expected to increase the likelihood of drivers exiting facing outward and neglecting to use both hands for support. Overall, the ideal arrangement of the steps encourages safe, inward-facing egress, but also provides reasonable support for outward-facing egress.
First Step Height -- As part of a recent study, drivers' preferences for the height of the first step were investigated by gathering subjective ratings as the step height was increased and decreased. The mean first step height in our survey of truck dimensions was 407 mm, well within the range that drivers view as acceptable. First step height is addressed directly in most standards, and it is apparent that a lower step height will be easier to use. However, given that only two steps will be used to ascend to a fixed cab floor height (a mean of 1230 mm), a low first step means a larger vertical distance between steps or between the top step and the cab. We believe that reducing the distance from the top step to the cab floor will help promote inward-facing egress by making it easier to transition from seated to standing on the steps. To accomplish that, the steps need to be higher from the ground. Hence, we recommend a 440 mm first-step height. Based on our analysis of driver preference data, we expect that about 70% of drivers will find this step height to be acceptable, even though it is about 15 mm higher than the fleet mean.
Second Step Height -- The second step should not be positioned midway between the ground and the cab floor. Rather, the second step should be positioned closer to the cab floor to allow drivers to easily and safely transition from the seat to the step. With the typical floor height above the ground of 1230 mm, setting the second step height at about twice the first step height will accomplish this objective. With a first step height set to 440 mm, the second step can be set to 880 mm, leaving an offset to the sill of 350 mm.
Step Depth -- The first step should be deep enough to allow even drivers with large feet to put the entire width of their shoes or boots on the step (125 mm), but ideally the step will be 150 mm deep to allow broad support from the toe through the midfoot for both ingress and egress. Some designs with reasonable step offset have restrictions on first step depth that result in awkward foot placements, particularly for outward-facing egress.
Horizontal Step Offset -- The horizontal offset between the edges of the first and second steps represents a tradeoff between maximizing the landing area of the second step (see below) and provide a comfortable and safe transition between the first and second steps. On egress, it is essential that the driver excuting an inward-facing egress maneuver is able to see the edge of the first step past the second step while standing with the left foot on the second step. A driver using an outward-facing egress manuever must be able to place a foot securely on the first step while the other foot remains in contact with the second step. Our field investigation of dimensions showed many vehicle models with step offsets between 60 and 100 mm. We recommend 100 mm for this dimension, and not less than 70 mm, although we recognize that it trades off with the second-step "landing" area objective, given a fixed overall cab width.
Second-Step "Landing" Area -- The transition from sitting to standing on the steps is the most physically challenging part of the egress motion. This is particularly the case for inward-facing egress, because the driver must pivot, sliding the right foot between the left foot and the seat. To encourage inward-facing egress, the second step should extend at the same height as far as possible toward the door opening. On many trucks, the area between the second step and the cab is filled by sloped trim. This trim may pose a slip hazard, but it also precludes the driver from using that area to facilitate the pivot necessary to use an inward-facing egress method.
Step Visibility -- Drivers can have difficulty seeing the steps when they exit, particularly if they use the preferred inward-facing egress method. Consequently, step features that improve visibility should be considered. Contrasting colors on the edges of the steps can improve visibility, particularly for grating-type steps. Lighting should also be considered. Drivers should be trained to check the condition of the steps before exiting (see Driver Training) and adequate lighting is needed to detect accumulated snow, ice, or other contaminants at night. Lights should also be aimed at the ground adjacent to the steps so that drivers can inspect the area before exiting.
Handhold placement is critical to provide stability during both ingress and egress. Most drivers use one or more handholds during routine IE movements, but the handholds are probably most critical when a foot slip occurs. Having both hands in contact with handholds probably reduces the consequences of foot slip.
Rear Handholds - The handholds at the rear of the door are the most important for ingress. A driver typically starts with the right hand on the rear handhold, although the hand position is not strongly affected by body dimensions. The left hand either grasps a handhold on the door or moves directly to the steering wheel (primarily taller drivers). An internal handhold (mounted inside the door opening) appears to be as effective for ingress as the external handhold, and results in lower hand forces for certain step offsets.
Current rear handhold designs appear to be generall adequate. We investigated several alternatives in our laboratory testing but did not find anything that was clearly superior. However, some design recommendations can be made based on our laboratory and pilot-test findings. To support preferred hand positions for ingress, the vertical extent of the handhold should extend at least from 1600 to 1800 mm above the ground, and preferrably from 1550 to 1850 mm (300 mm extent). This applies whether the handhold is internal or external. However, for egress, a larger upward vertical extent is desirable. As noted elsewhere in these recommendations, vehicle designers must assume that many drivers will exit facing away from the truck, in spite of training recommendations. For drivers who use a rearward- and outward-facing tactic, the external handhold is the critical hand support. The handhold should extend upward approximately to the shoulder height of the seated driver. This is difficult to achieve with an internal handhold, which is one reason to prefer an external handhold. External handholds must be mounted as close to the door opening as is practical, so that drivers do not have to move out of the cab before being able to grasp the handhold.
Internal handholds are preferred by some manufacturers because they are out of the weather (no rain, mud, ice, etc.) and because external handholds contribute to aerodynamic drag. However, internal handholds may not provide support for outward/rearward-facing egress that is equivalent to that provided by external handholds. More research is needed on this topic.
The friction provided by the handhold surface should be as high as possible. Many handholds are made from chromed metal, rather than knurled metal or other surfaces with high friction. A high-friction surface, for both bare and gloved hands, will reduce the risk of a hand slip and maximize the benefits of the hand grasp in the event of a foot slip.
Door Handholds – In our field research, we found that door-mounted handholds varied much more than aft handholds. A recessed handhold near the sill was common, as were bar-type handholds extending downward at various angles. On some trucks, a map pocket was used as a handhold. We observed a significant number of drivers exiting facing forward and outward, essentially facing the inner surface of the door. Although we would prefer that drivers face the truck when exiting, trucks should provide good support for this common "door-facing" behavior. Handholds are needed at least near the bottom of the door (about 1400 mm above the ground) and near the top of the door trim, below the window. The handholds should extend fore-aft sufficiently that the driver can place both hands on each of them if necessary (think ladder rungs). These handhold locations could be integrated into a single "Z"-shaped handhold or otherwise built into the trim, but both the high and low handholds are needed
The steps and handholds function together with the seat, cab openings, and other features of the truck to form an ingress/egress system. No single component of the system functions independently, and consequently the performance of the system must be assessed as a whole. This section presents some ideas for system assessment.
Support for Multiple Tactics –– An important conclusion from our research is that drivers use a wide range of tactics, particularly for egress, and a good IE system will maximize the safety for all of the likely tactics, even while nudging drivers toward safer, inward-facing tactics. Consequently, both virtual and physical assessments should include drivers using all three common egress tactics: inward-facing, outward/rearward-facing, and outward/forward-facing (door-facing). For each tactic, the truck should provide the ability to see the steps and ground and to use two hands throughout the motion.
Virtual Assessments –– The development of improved virtual assessment tools using digital human figure models is one of the objectives of our research. Although further work needs to be done in this area, figure models can currently be used to assess reach to handholds for each of the likely ingress and egress tactics. The primary objective is to verify clearance for vision and reach to handholds for both hands at all times for all common tactics.
Driver Assessments –– Testing with physical mockups is important, and experienced drivers can provide useful feedback. However, two points should be kept in mind when conducting and interpreting such studies. First, our data show that drivers behave differently (generally more conservatively) when then know they are being observed. For example, they are unlikely to volunteer the outward-facing tactics that our data show account for nearly half of egress events in the field. Second, driver's preferences for system layout may be contrary to the overall design objectives. For example, our laboratory data suggested that drivers prefer layouts that are more "stair-like" (larger horizontal offsets) whereas we believe that a larger top-step-to-sill "landing" area is more desirable. Similarly, drivers will tend to prefer shorter first-step heights, but the first-step height must be determined in the context of the overall system objectives.
Driver footwear is critical for ingress/egress safety. We have observed many drivers wearing footwear that does not provide a high-friction interface with the steps (cowboy boots) or provides little stability and support (flip-flops). We have not had the opportunity to research the influence of footwear on truck step interactions, but we make the following recommendations based on our understanding of the injury problem and solutions from other ergonomics domains.
1. Footwear should be fully enclosed (no open-toed sandals, for example) and well coupled to the foot (laces or velcro, not slip-on).
2. The sole should incorporate significant tread that can engage with the slip-resistant surface features on the steps. Slip resistance for truck steps is primarily about physical engagement between macroscopic feastures of the two surfaces, rather than micro-scale friction.
3. A pronounced heel pocket should be avoided if possible to reduce the risk of tripping in outward-facing egress. Many work boots have a gradual contour in the heel pocket that may present less of a tripping risk than a pronounced, sharp-edged heel.
Fleet safety personnel have reported dramatic reductions in slip-and-fall injuries after instituting and enforcing a footwear policy. We strongly recommend that fleet operators provide drivers with footwear based on the requirement that they wear it. In addition to reducing ingress/egress risk, good footwear can reduce the likelihood of many other types of slip-and-fall injuries.
Driver behavior is the most important determinant of injury risk in ingress and egress. A careful driver who is physically fit, attentive to the environment, and uses conservative tactics will have greatly reduced risk of injury. We have identified a number factors regarding driver behavior that may be amenable to training. We note that all drivers we interview report having been trained to use "three points of contact" on both ingress and egress, yet we observe only a small minority of ingress and egress events in which three points of contact are maintained. Drivers report having been trained to face the truck for both ingress and egress, yet we observe outward-facing egress about half of the time. These findings suggest that improving the effectiveness of driver training is an important area of emphasis.
The following sections address a series of factors that we believe are key behavioral determinants of injury risk. Driver training programs should emphasize these factors.
Slow Down –- The biomechanical data from the laboratory study show a strong effect of egress speed on ground reaction forces, which are related to both musculoskeletal loading and risk of slipping. Drivers who exit the vehicle quickly are likely to be at higher risk of falling, in part because their chances of catching themselves as a slip begins are smaller. Exiting quickly effectively eliminates the possibility of using three points of contact. Entering the cab quickly increases foot pressure on the steps and may increase the risk of slipping. Encourage drivers to check their environment before exiting and to go slow.
Facing Inward –– The behavioral data we have gathered show clearly that many drivers prefer to exit their trucks facing outward, even though drivers we interview indicate that they have been trained to face inward. Our kinematic and biomechanical analysis indicates that inward-facing egress involves more complex and difficult maneuvers to transition from sitting to standing on the steps. Many drivers are not able to see the steps when descending facing inward, which may create a perception of less control. We do not have evidence that slips and falls are less likely to happen facing inward than outward, but we believe that drivers are better able to catch themselves without injury when facing inward. This is particularly the case if the slip occurs when the first foot contacts the ground. With outward facing egress, most drivers have only a single hand in contact with the truck when the first foot hits the ground, and very little support force can be generated by the trailing foot. Consequently, a slip at that moment will have a high likelihood of resulting in a fall. In contrast, drivers more often able to use two handholds when facing inward, and may be better able to use their hands to protect themselves if a foot slips. The ground reaction forces are smaller for inward-facing egress, particularly for non-obese drivers, indicating lower speeds and greater body control on ground contact. Consequently, we recommend that driver training continue to emphasize inward-facing egress. System design should provide features that facilitate inward facing egress (see Systems recommendations on this page), but also provide the best available features for drivers who exit outward-facing.
Three Points of Contact –– Drivers report being trained to use three points of contact, but routinely violate that rule. Conceptually, moving one limb at a time reduces the risk if one of the other limbs slips, but maintaining three points of contact is perceived as being too slow for most drivers. (See the detailed section below on Three Points of Contact.) An alternative or complementary training approach would be to focus on using both hands every time. The system design must provide good handhold locations for a range of egress strategies, and not just the optimal inward-facing strategy.
Carrying Objects –– A driver carrying an object in one or both hands has little chance of maintaining three points of contact and is likely to fall if a foot or hand slips. Our behavioral observations show drivers carry an object when getting out of the cab as much as 15% of the time, although this can be expected to vary depending on the situation. Drivers should be trained to use both hands on the handholds when getting in and out, and to never carry objects. But truck designs must facilitate this practice by providing clean, dry places, off of the floor, for drivers to place objects prior to getting out that they can then access from the ground. On many trucks, only the floor of the cab is a convenient place to put a coffee cup, bag of trash, log book, or cell phone prior to exiting. Manufacturers should try to innovate in this area to reduce the risk drivers incur when they carry objects out of the cab.
Look and Don't Leap –– Drivers should be told not to jump from the truck, which produces much higher ground reaction forces and likely increases the risk of injury. With conventional cabs, few drivers jump from the cab floor, but a larger number jump from the first or second step. But drivers should also be trained to look carefully for hazards before exiting. Many injuries appear to be caused by slips due to unexpected ground surface conditions, such as ice, wet grass, or gravel. Drivers who take the time to look for hazards before exiting mayalso be more likely to move slowly, further reducing their risk. One challenge is that drivers cite the need to look for hazards as one reason for exiting facing outward. Drivers need to be trained to open the door, look for hazards, and then begin to exit.
Step Maintenance –- In our field investigations, we found many steps that were poorly maintained. Some steps were bent or worn, reducing the effectiveness of the slip-resistant features. Two types of maintenance should be emphasized: long-term attention to the overall functioning of the steps, particularly overt damage or wear, and immediate removal of any contaminants such as ice, snow, mud, or fuel. Using contaminant-resistant step designs will reduce the work required to maintain the steps.
Footwear Selection -- Footwear affects injury risk. (See Footwear section.) Drivers should be instructed on the appropriate types of footwear, and we recommend that fleets provide appropriate footwear. The cost of work shoes is a small fraction of the cost of an injury.
Physical Fitness –- As in many other areas of ergonomics, a low overall level of physical fitness probably increases a driver's risk of injury in ingress and egress. Drivers who are obese and have relatively weak muscles have a smaller safety margin, making it more likely that a slip will lead to a fall and a fall to injury. Drivers should be encouraged to be physically fit, but the laboratory research provides some evidence of compensatory behavior. Drivers who are obese tend to choose more conservative tactics, and move more slowly, so that their ground reaction forces (a measure of impact loading during egress) are not as high as they would be if they moved as quickly as non-obese drivers.
Drivers we interviewed report being trained on "three points of contact", which is taken to mean that two hands and one foot, or two feet and one hand, should be in contact with the truck at all times during ingress and egress. Another way to phrase this is that only one limb should move at one time, which means that all four limbs will be in contact with the truck intermittently. Here's a document we used in our laboratory testing to ensure that each driver had uniform exposure to the idea: Three Points of Contact Flyer.
Using three points of contact has apparent benefits because it provides a better likelihood of catching a slip before a fall, but its biggest strength may be that it results in unusually slow ingress and egress. So slow, in fact, that we infrequently observe people actually maintaining three points of contact, even when moving conservatively.
We hesitate to recommend doing away with the advice to use three points of contact, but given that it seems not to be producing the desired behavior, we suggest trying a complementary focus: Use Both Hands. Drivers will automatically use both feet when exiting, but often use only one hand. To use both hands, drivers will have to forego carrying objects, another relatively common and risky behavior. See the Driver Training section for more ideas on coaching drivers to enter and exit more safely.
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