NHTSA. Evaluation of Enhanced Brake Lights Surrogate Safety Metrics.            DOT HS 811 127. April 2009

Findings

The following summarizes key discussions and topic areas covered during the workshop.

Individual summaries with specific stakeholder groups are included in the sections that follow.

1. Any new signals should attempt to conform to (take advantage of) the existing brake

lamp frameworks/housings.

2. Standardize relevant (performance-related) functional system characteristics as much as possible. Triggering criteria, in particular, should be standardized.

3. System evaluations must also measure unintended consequences associated with the

introduction of any new signals. It is not sufficient to merely measure performance

benefits – also need to ensure signals are not introducing disbenefits.

4. Evaluations should include a control or comparison condition using a type of Forward

Collision Warning (e.g., audible alert). This will help to understand the relative benefits

of the various approaches. Assessments should also include closed-loop concepts,

particularly for stopped lead signals.

5. Both incandescent and LED configurations should be examined. However, standards

should be based on functional system characteristics and not technology.

What Makes for an Effective Signal?

ƒThe signal must capture (and/or redirect) the attention of a distracted driver.

ƒAn effective signal must also reduce or minimize unintended consequences.

ƒMust be intuitive and quickly understood by drivers.

Must be novel (effectiveness should be maintained over time).

ƒA signal that addresses the stop lead vehicle problem (which represents a big part of the

rear-end crash problem) would be a great enhancement.

ƒApproach should not rely on discrete changes (e.g., change in brightness, number lamps,

color, etc.) which may not be readily perceived (i.e., driver looking away at the time of

the change).

Candidate Concepts, Design Recommendations, and Recommended Research

ƒStaged approaches wherein the level of deceleration is communicated in some manner are not generally recommended.

Can add complexity and visual noise.

May fail to capture the attention of a distracted driver. Signals that rely on

discrete changes may not be perceived.

Misses the point; driver’s are very good at judging closing rate when looking

forward. Drivers are good at estimating time-to-collision (and the need to brake

hard) when they are looking forward even with short 1-sec glances to the vehicle

ahead.

Thus, an effective signal would capture and redirect the driver’s attention

to the forward roadway and not necessarily attempt to code level of deceleration.

ƒ Use the CHMSL to communicate deceleration signals

ƒFlash existing outboard brake lamps. May not need sustained or continuous flashing; a

short series of flashes may be sufficient to capture the driver’s attention. An option would

be to initially flash the CHMSL to gain driver’s attention followed by increase of

outboard lights to make use of looming (similar to Federal Highway Research Institute’s

recommendation).

ƒDesign system to allow more warning time (e.g., pre-charge system based on quick

accelerator pedal release)

ƒSystems that use looming cues may be more effective at night compared to daytime.

ƒSeveral strategies are possible following a hard deceleration event: transition to a

different signal (new cue) to indicate a stop, or continue with hard deceleration signal

(e.g., flashing lights).

o Need to determine how long the “stop” signal should remain active.

o Could also introduce a delay (e.g., wait 2-3 sec before activating signal after the

vehicle comes to a complete stop).

Closed-loop approaches are most relevant for the stopped lead vehicle application. If a

stopped lead signal is used, it should be part of an active, closed-loop system (e.g., signal

activates only if there is an approach vehicle in the lane, signal turns off when the

following vehicle is stopped, etc.). An inexpensive narrow beam sensor could be used to

detect the presence of an approaching vehicle.

ƒDeceleration should not be the only triggering criterion. Deceleration threshold may also

be adjusted for speed.

ƒNarrowing the width or focus of the visual signal may help to reduce annoyance and

unintended consequences from adjacent traffic

ƒExplore unintended consequences as part of the research paradigm.

ƒExamine effectiveness of signals under varying environments (day/night, low visibility,

etc.)

Current Concepts Being Considered

ƒMercedes-Benz, Adaptive Brake Light

- All three brake lamps flash (frequency 5 +/- 2 Hz ) during emergency

braking maneuver, hazards flash when vehicle stops following emergency

braking

- Triggers when vehicle speed > 31 mph AND at least one of the following:

ƒ Deceleration > 7m/s2 (0.7 g) and/or

ƒ Brake assist function active and/or

ƒ Electronic Stability Program detects a panic braking operation

ƒ Volvo, Emergency Brake Lamp

- “Warns other drivers of your emergency brake status. When the car’s

deceleration exceeds 0.7g, or when antilock brakes are activated, the brake

lights are accentuated by becoming brighter.”

ƒ BMW, Brake Force Display

“Enlarges the surface area of the brake lights as you increase the rate of

braking” (BMW 7-Series, 5-Series, 6-Series, X3)

Activates at 0.5g deceleration

ƒPeugeot

Automatic Illumination of Hazard Lights by Sharp Deceleration

Peugeot 607

Feasibility: Implementation Factors & Considerations

ƒCost, power and styling are always considerations in the design of a system. It would be

preferable to take advantage of and use the existing vehicle’s lighting

frameworks/housings.

o Flashing lamps, for example would be more preferable than the requirement to

enlarge the surface area. Flashing brake lamps is a simpler mechanization than the

others signal approaches discussed that can be more difficult to implement.

o Increasing the brightness of the lamps has the potential to overload the LED’s to

the point of failure.

ƒFlashing LED’s may leave a ghost image

ƒThe number of electrical drivers (e.g., lights) is one of the limiting factors (e.g., a light

bar with 8 lamps would be much more complicated to implement).

ƒ If flashing is used as a signal, ensure that the flash rate is not inducing epileptic seizures

in the population; suggest staying under 5 Hz flash rate.

Standardization

ƒFrom a Human Factors perspective, it would be desirable to standardize systems, since it

would increase consistency and enhance driver performance. Need to avoid having

different solutions or implementations across vehicle platforms.

ƒ The focus for standardization should relate to relevant performance or functional

characteristics of the system and not technology-based elements (e.g., LED versus

incandescent bulbs). Essentially, performance-related aspects should be standardized, but

no component that affects styling. The following key characteristics should be

considered for standardization:

o activation speed,

o deceleration trigger,

o warning strategy or approach,

o the type of cue used to signal hard deceleration events

o the rate of flashing

o location of brake lamps, etc.