Research
The most dangerous thing in the trunk
Police cars caught fire often enough that Tucson’s crime lab and fire investigators ran experiments to find out why. The answer was the road flares — and a federal study then tested every alternative and found none of them worked as well.
A box of magnesium fusees has ridden in the boot of almost every American patrol car for eighty years. They are the least glamorous thing in the vehicle and the one nobody writes about.
They are also, on the evidence of this study, the most hazardous object an officer routinely carries — to the officer, to the water supply, and to the car itself.
The trunk fires
Start with the part that belongs to this site rather than to environmental science.
Patrol cars were catching fire. Enough of them that the Tucson Police Department’s Crime Lab and Arson Detective Unit, working with Tucson Fire Department investigators, ran a series of experiments to establish why.
The conclusion was that under the right set of circumstances, flares might be unintentionally ignited in officers’ cars. A city-wide memo followed, and its instructions tell you exactly what officers had been doing:
Flares should never be readied or prepared in advance in anticipation of need. Igniter and striker caps must remain intact on the flare until it is needed. Flares should be kept in their original box or in a container, preventing migration and minimising movement.
A striker cap, a rough surface and a car spending its shift accelerating, braking and cornering. Loose flares in a moving boot were striking themselves.
What the flare does to everything else
The study’s account of the traditional fusee is unsparing, and it is worth reading because none of it is controversial — it is simply not discussed.
To the officer. The flares burn at high temperature, generate noxious smoke and fumes that can overwhelm the user, and the burning end can cause serious burns from molten magnesium. Strontium nitrate, which produces the colour, irritates skin, eyes and mucous membranes; so does potassium perchlorate. The safety guidance is to work in a well-ventilated area and wash your face and hands thoroughly before handling food.
To the road. They burn for fifteen to thirty minutes, leaving the officer with a hot melted fusee to dispose of. The study’s focus group found that officers frequently kick them to the side of the road, leaving sharp metal spikes behind as a future hazard. Many agencies do not address disposal in policy at all.
To the water. The Santa Clara Valley Water District concluded that incendiary flares can be a significant source of perchlorate contamination of both surface and groundwater — and that unburned flares improperly disposed of can contaminate water with perchlorate up to 2,000 times more than completely burnt ones.
The Canadian Police Research Centre had listed the problems as early as 1992: not environmentally safe; an extreme hazard at oil and gas spills; capable of damaging uniforms and equipment; spike remnants must be collected after every burn; striker and cap must be disposed of; difficult to extinguish; offensive and noxious fumes; a case of them is heavy and hard to store; and their brilliance at night is itself a hazard, often distracting oncoming drivers.
So they tested the alternatives
Researchers at Florida Gulf Coast University, funded by the National Institute of Justice, assembled the commercially available replacements — PowerFlare, Cyalume lightsticks, TurboFlare, ProFlare, Flare Alert Beacon Pro, Tektite traffic strobes — plus reflective and collapsible cones, and tested them against traditional Orion road flares.
The method was more careful than the subject might suggest. A nine-point visibility scale was built using a modified Delphi process, with the traditional highway flare anchored at 9. Eight research assistants were trained over roughly 42 hours. The systems were then scored at distance intervals up to and including one mile, in scenarios derived from real driver reaction and stopping distances.
Those stopping distances are the reason any of it matters. At 70 mph a vehicle travels 205 feet during the reaction period alone, then another 245 while braking — 450 feet in total. Whatever marks the hazard has to be legible well before that.
The finding nobody wanted
The traditional magnesium highway flare — the burning, poisoning, self-igniting one — was found to be highly visible and scored well in all scenarios.
The chemical and electric alternatives were less visible than the highway flare when deployed at ground level.
That is the whole problem in one sentence. Everything wrong with the fusee is real and documented, and the thing it is supposed to be replaced by does not do the job as well lying on tarmac.
The supporting history says the same. A 1971 study on a ten-mile driving course found that in daylight a warning triangle, a triangle with a flag and a road flare were all seen equally well. At night the flare added six seconds of advance warning. At 40 mph it was visible for 13.66 seconds against the triangle’s 7.07 — nearly double.
The flare is dangerous because it burns. It works because it burns. Nobody has got round that.
Two findings that belong with the light bar story
The literature review contains two results that connect directly to the lighting research elsewhere on this site.
Green is the most visible colour on a highway scene — confirmed across several studies, and in one survey of mine rescue team members, 73 percent identified green chemical lightsticks as the most dominant colour seen. But green, the study notes, is not immediately recognised by drivers as a signal to proceed with caution. The same gap between visibility and recognition that runs through every conspicuity study here.
And a 2005 test by Lieutenant Wells of the Florida Highway Patrol found that a bi-coloured red-and-blue light bar provided the greatest illumination, and that breaking the light pattern into random flashes made the cruiser more visible and gained attention more easily — without altering oncoming drivers’ depth perception.
Red and blue, again, arrived at independently. That is now three separate studies on this site reaching it from three directions.
Why this is a police car story
Because the boot of a patrol car is the one piece of it nobody restores accurately, and this explains what was actually in there and how it was handled.
Flares in their original box, caps intact, never prepared in advance — that instruction exists because cars burned. The spent fusees at the roadside, the spikes in the verge, the officer washing his hands before eating: that is the daily reality of a piece of equipment that appears in no test book and no evaluation programme.
Michigan measures the brakes to a hundredth of a second. Nobody was measuring the thing most likely to set the car on fire.