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Public Roads - Sept/Oct 1997

Interactive Highway Safety Design Model: Design Consistency Module

by Raymond A. Krammes


The Federal Highway Administration (FHWA) has a high-priority research program to develop and implement the Interactive Highway Safety Design Model (IHSDM). IHSDM is an integrated system of modules that highway planners and designers can use to evaluate the safety of highway geometric design alternatives within a computer-aided design (CAD) environment.

This article focuses on design consistency, one of five modules that comprise IHSDM. Before turning to design consistency, however, it is important to understand the overall structure of IHSDM. Then, the concept and measures of design consistency and its evaluation within IHSDM will be discussed. To conclude, an example is given to illustrate the prototype Design Consistency Module.


In its current form, available information on the safety effects of highway planning and design decisions is not readily usable to evaluate and compare design alternatives. The lack of convenient evaluation tools limits a designer's ability to detect potential safety problems in design, to select safety cost-effective design parameters, and "to compare the safety of various alternatives or to optimize the safety of a particular design."1 From a broader perspective, it is difficult to assess tradeoffs between safety versus social, environmental, and economic impacts. As a result, differences in safety performance may not be given due consideration in deciding among design alternatives.

IHSDM is envisioned as a computer-based tool that facilitates evaluation of the safety implications of design alternatives throughout the planning, design, and review phases of typical highway construction or reconstruction. For ease of use, it is being developed in a CAD environment, in which most design work is currently performed. The initial development efforts are restricted to two-lane rural highways, such as the one pictured above. Two-lane rural highways are the largest single class of highways, representing approximately two-thirds of all federal-aid highways. Because of their age, condition, and crash experience, they are common targets for improvement projects. As such, they are also a logical initial focus for IHSDM. A second phase of IHSDM development will add the capability to evaluate multilane design alternatives.

As illustrated in figure 1, IHSDM is structured in five modules:

  • Accident Analysis Module will consist of three models: one that estimates the number and severity of crashes on specified highway segments; a second that performs safety benefit-versus-cost analyses of alternative roadside designs; and a third that uses an expert systems approach to evaluate intersection designs, identify geometric deficiencies that may impact safety, and suggest improvements that correct these deficiencies.
  • Design Consistency Module, as described in more detail in this article, will evaluate the consistency of a design relative to drivers' speed expectations.
  • Driver/Vehicle Module will consist of a Driver Performance Model linked to a Vehicle Dynamics Model, which will permit the designer to evaluate how various drivers would operate a given vehicle (e.g., passenger car or tractor-trailer) through a design and identify whether conditions exist that could result in loss of vehicle control (e.g., skidding or rollover).
  • Traffic Analysis Module will use existing microscopic traffic simulation models to estimate likely operating conditions on a design under current and projected traffic volumes.
  • Policy Review Module will evaluate design elements for conformance with established design policies (AASHTO, state, or local), identify elements that deviate from those policies, and aid the designer in documenting the safety implications of those deviations.

IHSDM provides an integrated collection of evaluation tools that estimate safety-related measures of effectiveness, identify areas for improvement, and compare design alternatives. A users group representing state departments of transportation and FHWA field offices provides periodic input to ensure that these tools are responsive to the needs of the user community. FHWA is also working with civil design software vendors through cooperative research and development agreements so that IHSDM can be integrated into those software packages for delivery to highway planners and designers.

This article is the third in a series on IHSDM in Public Roads.2,3 Future articles will describe other modules.

Design Consistency

Drivers are inclined to interpret and react to roadway features or situations as if those situations were similar to what they have experienced previously, whether or not they actually are similar. As a result, drivers are more likely to become confused and, possibly, commit errors at features that violate their inclination -- or, in human factors terminology, driver expectancy -- than at features that are consistent with their inclinations. Therefore, design consistency, which refers to a roadway design's conformance with driver expectancy, is an important roadway characteristic from a safety perspective.

Driver errors related to inconsistent geometric features typically occur in guidance (i.e., speed and path) decisions. Current design policy specifies that a design speed should be selected and applied along a roadway in order to avoid geometric features that require unexpected and, therefore, error-prone adjustments in speed.

It has become apparent, however, that the design-speed concept, as currently applied, is not sufficient to ensure design consistency measured in terms of the uniformity of operating speeds along a roadway.4 A roadway's design speed is the maximum speed at which a driver can operate uniformly through all geometric features of the roadway without deviating from assumed design criteria. Thus, a single design element (e.g., a sharp curve) may result in a relatively low design speed when, in fact, drivers may be able to operate at speeds higher than the design speed on the rest of the roadway without exceeding any design criteria. Since most drivers travel as fast as they feel comfortable and slow down only where necessary, rural highways with lower design speeds exhibit uneven operating speed profiles. That is, drivers accelerate to their desired speed on tangents and gentle curves and decelerate only on sharper curves.

Crash studies have shown that the larger the speed reduction required from the preceding tangent to the subsequent curve, the higher the crash rate on the curve -- i.e., the higher the required speed reduction, the more likely it is that some drivers will not reduce their speed as much as required. Figure 2 summarizes relative crash rates on curves that require speed reductions compared to curves that do not require speed reductions.5 For example, the expected crash rate on curves that require a 10-km/h speed reduction is three times the rate on curves that require no speed reduction.

Therefore, the primary measure of design consistency for two-lane rural highways is the expected reduction in 85th percentile speed between the approach tangent and the middle of a curve. Expected speed reductions are derived from a speed-profile model that estimates speeds at each point along a roadway as a function of the geometry of the roadway. The model combines estimated 85th percentile speeds on curves and desired speeds on long tangents with estimated deceleration and acceleration rates entering and departing curves to produce the speed profile.

A model has been calibrated for passenger vehicles in level and rolling terrain (i.e., vertical grades less than 5 percent) as a function of horizontal alignment geometry.4 The calibration was based on more than 20,000 passenger vehicle speed measurements at 138 curves and 78 tangents on 29 two-lane rural highways in five states. Research is underway to expand the model to consider a wider range of horizontal and vertical alignment conditions and to calibrate it for other vehicle types (i.e., trucks and recreational vehicles).6

Prototype Design Consistency Module

FHWA's Geometric Design Laboratory at the Turner-Fairbank Highway Research Center has developed a prototype of the Design Consistency Module. To illustrate the use of the module, it was applied to the two-lane rural highway shown in figure 3. This plan view shows the 1.1-km highway's horizontal alignment, which consists of four tangent segments and three circular curves. The alignment has a 60-km/h design speed.

The Design Consistency Module is executed through a dialog box within the CAD environment, as illustrated in figure 4. The first step in evaluating design consistency is to "create" the speed profile by "clicking" that button.

Figure 5 shows the resulting, estimated 85th percentile speed profile. It has the characteristic saw-toothed shape of operating speeds along a highway whose design speed is less than most drivers' desired speed. The profile includes three curves -- represented by horizontal lines that indicate the speed on each curve based upon its sharpness -- and two intervening tangents -- represented by the spikes between the curves that indicate acceleration out of one curve and deceleration into the next curve.

To facilitate interpretation of these results, the user may request options in the dialog box to "color-code" or "flag" the speed profile. In figure 6, the color-coding of the speed profile indicates how much 85th percentile speeds deviate from the design speed. Green represents 85th percentile speeds that differ from the design speed by no more than 10 km/h, yellow indicates a difference between 10 and 20 km/h, and red indicates a difference of more than 20 km/h. Figure 6 illustrates that the design of the roadway should be expected to elicit 85th percentile speeds that exceed the design speed by more than 20 km/h over much of its length.

In figure 7, the colored flags indicate the magnitude of differences in 85th percentile speeds from each tangent to the succeeding curve. Green flags indicate speed changes no more than 10 km/h, yellow indicates a change between 10 and 20 km/h, and red indicates a change greater than 20 km/h. According to figure 2, the expected crash rate on the curve with the red flag is more than six times the rate of a curve requiring no speed reduction.

The information provided in figures 6 and 7 allows planners and designers to double-check their design-speed assumption. Figure 6 alerts designers that 85th percentile speeds are likely to be significantly higher than the assumed design speed, and figure 7 identifies with a red flag the curve where resulting safety impacts would probably be greatest. Since speed influences many alignment and cross-section design decisions, a large discrepancy between design and 85th percentile speeds would prompt the designer to consider design modifications that increase the design speed and/or decrease likely 85th percentile speeds. If the assumed design speed is, in fact, the appropriate target speed considering the roadway's function, then the designer would seek design modifications that would influence drivers to operate more uniformly at that target speed -- for example, sharpening the flatter curves and/or shortening tangent lengths. If a higher design speed is desirable, then the designer would need to consider another set of modifications to accommodate a higher speed -- for example, flattening sharper curves, increasing sight distance, widening the clear zone, and flattening roadside slopes. Whereas these modifications may be feasible for a new roadway, they would be impractical on many existing roadways. If social, economic, environmental, or other constraints preclude these modifications, then the designer should consider other actions to mitigate potential safety impacts. Possible actions include widening or paving shoulders on sharper curves, removing or relocating roadside obstacles, relocating or increasing sight-distance triangles at intersecting roadways on curves, and providing supplementary signing and pavement marking treatments.


The Design Consistency Module provides highway planners and designers with feedback about the reasonableness of their design-speed assumption. The color-coding and flagging alert designers to design elements that require extra attention. Designers could respond to this feedback by changing the design speed, adjusting the design to smooth out the speed profile, and/or taking other actions to improve safety. In contrast, current design procedures provide no feedback that highlights potential safety problems; as a result, designers may not realize opportunities to make feasible refinements that would improve safety.

A prototype of the Design Consistency Module is currently available. Ongoing research, scheduled for completion in 1999, will validate the module and expand its scope to consider a wider range of horizontal and vertical alignment conditions and to provide speed estimates for trucks and recreational vehicles. The expanded module will be beta-tested by members of the IHSDM users group, refined as necessary, and made available for stand-alone use and for incorporation into commercial civil design software packages.

Future Public Roads articles will review the Accident Analysis, Driver/Vehicle, Policy Review, and Traffic Analysis Modules of IHSDM, which provide additional measures of safety effectiveness. Development efforts are in progress for each module, and versions will be released as individual modules are completed. The scheduled release date for the full IHSDM, consisting of all five modules, is 2002.


1. Jeffrey E. Paniati and Justin True. "Interactive Highway Safety Design Model (IHSDM): Designing Highways with Safety in Mind." Transportation Research Circular 453, February 1996, pp. 55-60.

2. Jerry A. Reagan. "The Interactive Highway Safety Design Model: Designing for Safety by Analyzing Road Geometrics." Public Roads, Vol. 58, No. 1, Federal Highway Administration, Washington, D.C., Summer 1994.

3. Harry Lum and Jerry A. Reagan. "Interactive Highway Safety Design Model: Accident Predictive Model." Public Roads, Vol. 58, No. 3, Federal Highway Administration, Washington, D.C., Winter 1995.

4. R. A. Krammes, R. Q. Brackett, M. A. Shafer, J. L. Ottesen, I. B. Anderson, K. L. Fink, K. M. Collins, O. J. Pendleton, and C. J. Messer. Horizontal Alignment Design Consistency for Rural Two-Lane Highways. Report No. FHWA-RD-94-034, Federal Highway Administration, Washington, D.C., January 1995.

5. A. Voigt. Evaluation of Alternative Horizontal Curve Design Approaches on Rural Two-Lane Highways. Report No. TTI-04690-3, Texas Transportation Institute, College Station, Texas, August 1996.

6. "Design Consistency Evaluation Module for the Interactive Highway Safety Design Model." Contract No. DTFH61-95-C-00084.

Raymond A. Krammes is a senior highway research engineer in FHWA's Office of Safety and Traffic Operations Research and Development, Safety Design Division. Before joining FHWA in 1997, he spent 11 years conducting research at the Texas Transportation Institute and teaching transportation engineering in the Civil Engineering Department at Texas A&M University. He received bachelor's, master's, and doctoral degrees in civil engineering from Pennsylvania State University. He is registered as a professional engineer in the state of Texas.