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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
July 18, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Simulated On-the-Road Emergencies...
Planning and Designing Rail-Trails on Abandoned Rail Lines
Report Compares Methods of Analyzing Ramp-Free Junctions
Study Spotlights Railroad-Highway Grade Crossing Warning Systems

Simulated On-the-Road Emergencies Used to Test Stopping Sight Distance Assumptions

For 55 years, a concept known as stopping sight distance (SSD) has figured significantly in the design of roadways. The American Association of State Highway Officials (AASHO, known now as AASHTO) first proposed the common model for predicting SSD, and "the model remains a simple chaining of constant deceleration after an allowance of lag time for the driver to detect a hazard and initiate the braking maneuver." While the model itself has remained relatively unchanged, the term "lag allowance" has been changed to "brake reaction time" (the estimate of the perception-reaction time (PRT) for braking to occur), and the initial time of 2 to 3 seconds has been changed to a constant 2.5 seconds.

Recent research suggests, however, that this time-tested model is ready for change, to a model "that has its roots not in theory or engineering judgement but in actual performance of real people in real vehicles on real roads." Like virtually all experimental researchers studying human behavior, those researching driver behavior are confronted by the problem of subject awareness--the idea that subjects in a research experiment behave differently when they know they are being observed and evaluated. To overcome that problem, researchers have tried pure "covert observation" (when drivers do not know they are being observed), but success in measuring driver performance has been limited at best.

The new braking research prompts the question: "How do real drivers behave in emergency situations when SSD is a significant factor?" For an answer, researchers launched "a comprehensive braking performance study . . . . [whose] objective was to evaluate driver braking response to an unexpected hazard encountered in the roadway--one that would tend to trigger an extreme braking maneuver . . . ." In their article entitled "Measuring Driver Performance in Braking Maneuvers" (Transportation Research Record 1550), Rodger J. Koppa, Daniel B. Fambro, and Richard A. Zimmer highlighted the methodology of that study.


The instrumentation device used in the study at hand measured driver braking response to a surprise hazard. The instrument had significant advantages over previous designs. It proved reliable and could be quickly and easily installed and removed (in less than 30 minutes and 10 minutes, respectively). The experimenter (onboard) signaled the driver of a hazard by way of a concealed "button box." Signals were either a red LED light on top of the dash or a horn.

All data were processed into a Compaq laptop computer through an interfacing data acquisition unit (the DaqBook/100 manufactured by IOtech, Inc., of Cleveland, Ohio). For each trial run, the Compaq Model SLT 386z20 laptop computer performed a data acquisition program. After entering the date, subject, etc., the researcher logged an initiation command to begin data acquisition and later a second command to end the experiment. The program (written in QuickBasic) then automatically wrote to a hard-drive file. The instrument was conveniently powered from the host vehicle's 12-volt electrical supply, an optimum source that ensured no power loss. Car owners were comfortable having the instrumentation installed on their own vehicles.

Researchers made more than 3,000 test runs, with a significant success rate; they discarded only about 100 runs because of instrumentation or computer problems. A variety of braking scenarios were studied using nine test drivers; a later phase of the study involved volunteer drivers using either their own or a test vehicle.

Surprise Braking Maneuvers

Drivers were given a few practice runs to acquaint themselves with the course and its conditions before experiencing "a completely unexpected barrier that suddenly sprang up from the pavement in their path." The "barrier" hung from an arm concealed in a two-inch (5-cm) wide trench in the pavement. Attached to the arm was a monofilament line. When pulled tight, the line unfolded a piece of cloth displaying four stop signs. Researchers activated the barrier device with a garage door opener, and the hydraulic unit that operated the equipment was hidden behind traffic barrels at the side of the road.

The drivers' approach was at 55 miles per hour (88.5 km/hr), with the barrier timed to be visible 210 feet (64 m) ahead of the vehicle "using a 1-sec latency [response time] and pavement friction of 0.80." By allowing such a short time in which to respond, researchers hoped drivers would brake rather than try to evade the barrier. The barrier gave way without damage to the vehicle if a driver hit it. Ten of the drivers did just that, and two drivers showed no reaction and drove right through the barrier--one mistook it for a finish line; the other had no explanation.

Expected Braking Maneuvers

All drivers experienced the unexpected braking scenario before researchers exposed them to expected braking scenarios. Twenty-six drivers used a test vehicle from Texas Transportation Institute (TTI), and twelve used their own vehicles. Males and females of various ages participated. For the expected braking-maneuver experiments, subjects drove 55 mph. Researchers asked them to stop as quickly as possible if they saw the bright LED light come on--an event that might or might not occur. Both wet and dry pavement surfaces were used, as well as straight roadway and horizontal curves. Drivers knew only that braking was likely to occur on most trials.

On-Road Braking Maneuvers

A section of rural two-lane roadway ("asphaltic concrete in moderate to poor condition") was used for the on-road portion of the testing. Researchers asked drivers to drive as they normally would on such a road. A pickup truck was parked perpendicular to the road in the entrance drive to a pasture; the pickup was loaded with cardboard drums. At first, drivers drove past the pickup. Later they were instructed to turn around and travel back the same way. Upon a signal from the test vehicle, one barrel rolled from the pickup onto the roadway. To lend credibility to the scenario, a researcher posing as a farmer was unloading the barrels when this "accident" occurred. The barrel was released (on the driver's right side) when the test vehicle was 75 feet from the pickup. The posted speed in this section of the roadway was 45 mph, which again allowed approximately a one-second response time for the driver to begin braking.


Table 1 below shows results for the three test situations. The table indicates mean PRTs ("lag from first onset of signal or appearance of obstacle to initiation of the braking or other response") and gives a baseline (the baseline condition involved a stationary-vehicle test in which drivers were told to apply the brakes as soon as they saw the LED).

TABLE 1     PRT Comparisons
Condition Car No. Mean STD 25th* 95th* 99th*
Baseline Either 38 0.47 s 0.09 0.38 0.78 0.89
Expected TTI 38 0.60 0.18 0.42 1.05 1.22
Surprise TTI 38 0.82 0.18 0.64 1.23 1.39
Expected Own 12 0.62 0.21 0.29 1.36 1.63
Surprise Own 10 1.04 0.27 0.64 1.83 2.12
On Road Own 11 1.10 0.21 0.80 1.69 1.91
COMSIS (4)** Own 56 1.51 0.39 1.14 2.30 2.61
* Percent tolerance estimates conservative since distribution is truncated and positively skewed.
** A 1995 study of the perception-reaction time of older drivers.

Figure 1 "takes the 95th percentile estimates and plots them as a function of experimental condition." Results reveal that drivers tended to be more conservative with their own cars than they were with test vehicles.

FIGURE 1     PRT under test conditions


** A 1995 study of the perception-reaction time of older drivers.

Of note are the shorter PRTs for the on-road scenario than the surprise one. Drivers later said the barricade in the surprise scenario was startling, but they knew it was controlled by the test. However, none of the drivers thought the cardboard-drum situation was controlled. Perhaps drivers had a heightened awareness and response time to a "real," as opposed to contrived, obstacle; in addition, they may have been more alert and reactive because they knew they were being tested. Also of note is that all of the 95th percentile estimates and all but one of the 99th percentile estimates fell within the 2.5-second PRT from the AASHTO model.


Steady Deceleration

Under expected-stop conditions, research shows drivers generally exert an average steady braking force of -0.35 g (g = acceleration of gravity; 1 g is about 32 feet per second per second). This amount of braking force seems comfortable for drivers. Computing constant braking force (deceleration) over the length of the stopping distance in these tests, researchers found that under Surprise conditions drivers maintained an average of -0.63 g (standard deviation 0.08) in TTI vehicles and -0.55 g (standard deviation 0.07 g) in their own vehicles.

(Editor's note: Many wet pavement surfaces will not provide the high levels of braking force cited above. AASHTO assumes a braking force (coefficient of friction) of 0.28 in its formula for computing stopping sight distance at 70 mph and a pavement friction of 0.40 for 20 mph.)

Table 2 compares steady braking performance for test subjects under Expected and Surprise conditions while driving TTI vehicles or their own vehicles.

TABLE 2      Steady Braking Performance Comparison
Condition Car No. Mean STD 25th* 95th* 99th*
Expected TTI 38 -0.53 g 0.08 -0.61 -0.36 -0.29
Surprise TTI 38 -0.63 0.08 -0.71 -0.38 -0.29
Expected Own 12 -0.54 0.11 -0.69 -0.24 -0.13
Surprise Own 10 -0.55 0.07 -0.65 -0.35 -0.27
* Percent tolerance estimates conservative since distribution is truncated and positively skewed.

Maximum Braking

Analysis of a typical braking run revealed that drivers reached a maximum braking force on wet pavement of almost -0.6 g within 5 seconds. By scanning data files, the researchers found drivers of TTI cars averaged -0.91 g (with a standard deviation of 0.08 g) maximum deceleration, while those driving their own vehicles averaged a peak deceleration of -0.74 g (with a standard deviation of 0.09 g). The authors speculated that drivers may have perceived the braking tests as "severe" and, consequently, avoided putting that kind of wear and tear on their own vehicles. On the other hand, they were more willing to subject test vehicles to severe treatment.


The instrumentation used in this study proved practical, reliable, and easy to install; in addition, "the data for each maneuver sequence is amenable to analyses for a wide variety of purposes." Of particular interest are "the considerable differences" between performances when drivers were at the wheel of their own car as opposed to driving a test vehicle. Drivers may inherently be more conservative with their own vehicles, but the unfamiliarity of a new vehicle may also come into play. These differences "may disappear under simulated (but seen to be genuine) emergency conditions," and future studies are warranted to make further determinations. Expendable research vehicles might be well suited to "extreme maneuver studies," but a driver's own vehicle is the vehicle of choice when studying everyday driving conditions. The driver's own vehicle provides the benefits of both a familiar vehicle and a natural setting--the "covert ideal" for this type of empirical research.

Copyright © 1997 by TranSafety, Inc.

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