Yahoo Answers: Answers and Comments for A person driving her car at 54 km/h approaches an intersection just as the traffic light turns yellow.? [Physics]
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From suzy1231
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Fri, 09 Feb 2018 22:48:03 +0000
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Yahoo Answers: Answers and Comments for A person driving her car at 54 km/h approaches an intersection just as the traffic light turns yellow.? [Physics]
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https://uk.answers.yahoo.com/question/index?qid=20180209224803AAA6tIi
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From oubaas: A person driving her car at 54 km/h approaches...
https://uk.answers.yahoo.com/question/index?qid=20180209224803AAA6tIi
https://uk.answers.yahoo.com/question/index?qid=20180209224803AAA6tIi
Sat, 10 Feb 2018 07:09:49 +0000
A person driving her car at 54 km/h approaches an intersection just as the traffic light turns yellow.She knows that the yellow light lasts only 2.0 s before turning to red, and she is 30 m away from the near side of the intersection. The intersection is 15 m wide. Her car's maximum deceleration is 5.0 m/s2, whereas it can accelerate from 54 km/h to 70 km/h in 4.8 s. Ignore the length of her car and her reaction time.
If she hits the brakes, how far (d) will she travel before stopping?
Vin = 54/3,6 = 15.00 m/sec
d = (Vo^2Vin^2)/2a = (015^2)/(2*5) = 225/10 = 22.5 m ....safely well before the stopping line
If she hits the gas instead, how far (d1) will she travel before the light turns red?
Vfin = 15 +(7054)*2/(4.8*3.6) = 16.85 m/sec
d1 = (15+16.85)/2*2 = 31.85 m ....just two meters after the stop line, i.e. inside the cross area
she'd better braking

From Andrew Smith: The first thing to note is that as the speed i...
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Sat, 10 Feb 2018 03:07:46 +0000
The first thing to note is that as the speed is 15 m/s then with 2 seconds warning, then even with expert reflexes this only gives 1.5 s in which to act. ie cars would need to be able to stop with 10 m/s^2 to stop in time.
If a person is even slightly distracted then 1 s would elapse prior to doing anything at all.
Our rules for road design indicate that cars should not be required to brake or corner at more than 3 m/s^2
Clearly these lights fail good design specifications.
So to your question.
v^2 = 2as > s = v^2 / (2a) = 225/(10) = 22.5 m
7054 km/hr >16 km/hr > a = 4.44/4.8 m/s^2 = 0.93 m/s^2
in 2 seconds the speed will be 15 + 0.93 * 2 = 16.85 m/s
The AVERAGE speed is (15+16.85) / 2 and the distance is then ( 15+16.85) / 2 * 2 = 31 m
So in this exercise she can either brake and stop in time or accelerate and START to cross the intersection before the lights go red.