Mount your smartphone in the dash of your car and it could join a network of smartphones that watch traffic lights and tell you how fast to drive to get to the next traffic light without wasting gas. Researchers from MIT and Princeton say the technology, called SignalGuru, helped drivers cut fuel consumption by 20% in tests done in Cambridge, Mass. The see-all cameras could also note fuel prices and available parking spaces or, who knows, who’s looking hot in lines outside crowded clubs.

SignalGuru plays off the increasing number of drivers who buy a windshield-mount bracket for their smartphones. The brackets typically allow the smartphone’s back-side camera, now facing forward, to get a wide view of the road ahead, and they have enough resolution to see traffic lights and roadside signs. Software linked to the smartphones recognizes traffic lights and reports back their status and location. Based on reports from other cameras, SignalGuru tells the motorist how fast to arrive at the next light as it turns green.

The researchers also tested the camera in Singapore, where the duration of traffic lights is already paced to traffic flow, so you’ve got two smart systems dueling. In Cambridge with dumb (fixed-duration) traffic lights, SignalGuru could predict the timing of the next light with an error of two-thirds of a second. In Singapore, the error increased to just over one second, and two seconds in a densely populated area. But even with an accuracy of two and a half seconds (early or late to the next signal), says Emmanouil Koukoumidis, a visiting researcher at MIT from Princeton who led the project, “[SignalGuru] could very well help you avoid stopping at an intersection.” A traffic signal typically runs 20 to 30 seconds.

In the SignalGuru research project, feedback to drivers came via prompts on the smartphone displays. In a commercial application, it might be audio prompts, Koukoumidis says. Or not if they get feedback from real-world drivers, who often dislike being told how to drive when others are in the car and can hear the instructions. The MIT-Princeton crew modeled the effect of telling motorists to speed up as well as maintain speed or slow down to catch the next light, but “we think this application [go faster] is not a safe thing to have.” Merely the way a lot of people drive when they see a light turn yellow. In Europe, automakers even use the lane departure warning camera to read speed limit signs and construction warnings. They say the variability of US traffic signs from state to state makes it too difficult. (Like a sign reading “Speed Limit 65 Trucks 55” is difficult?)

Smartphones might be an interim solution until every car has an integrated camera. On-board cameras exist already in cars with lane departure warning or lane keep assist systems. A video camera embedded in the rear view mirror provides a view of the road ahead. A black box chock full of microprocessors figures out the lane markings — easy on an long stretch of interstate, less easy when the lane markings stop on one side at an exit or merge point — and if you veer across the lane markings and the turn signal isn’t flashing, the car sounds an alert: a beep heard by everyone on the more annoying cars, a vibration in the steering wheel or seat on the more polite cars. The same camera already does double duty on cars with variable intermittent wipers. It senses the blurriness of the image, the result of more or less rain on the windshield, and adjusts the wipers accordingly.

The downside to integrated cameras is sloth. Not in tracking traffic lights but in the automaker pondering how it integrates with everything else in the car, then kicking it over to legal to make sure nobody gets sued, then running more reliability tests, by which time the iPhone 10 and Droid 9 have already come and gone. Until automakers have more open interfaces — with appropriate firewalls and safeguards — their technology will be more expensive and less timely. (On-board navigation $1,500, TomTom $200.)

The MIT researchers say that SignalGuru might have other applications, too. The cameras could track fuel prices based on signs at gas stations, notice free parking spaces, or track the progress of mass transit vehicles (for others not in cars). All have promise and all have issues: In the US, stations often post three or four prices, so the software would have to be smart enough to know which is regular, mid-grade, premium, and diesel, and whether that’s the cash or credit card price. Similarly, it would be easy to find parking spaces on the street, harder to track the ones that are no-parking because of restrictions or fire hydrants, and near impossible to track the capacity of off-street or garage parking. For that, there are others apps being developed.

Buying tip: If you spend a heap of money for a smartphone, don’t be cheap on the phone mount. Pay the extra $40-$60 for a dedicated dash mount, meaning one specifically for the Droid 3 or iPhone 4. (That’s a Droid X in its Motorola cradle, at top.) It typically has a built-in power connector, so all you need to do is drop the phone in the bracket and you’re ready. With cheaper, one-bracket-fits-all dash mounts, you have to adjusts the fit each time and then plug in the power cable. Life is too short.

For more, see the MIT release