Final answer:
To safely cross the road, a car traveling at 60 km/hr (16.67 m/s) should be more than 13.6 car-lengths away, as it would cover around 47.60 meters while a person walks 4 meters at a typical walking speed.
Step-by-step explanation:
When considering how far away a car must be before it is safe to cross, we must refer to the scenario provided where the speed limit is 60 km/hr. First, convert this speed to meters per second to make calculations more intuitive. Since 60 km/hr is equivalent to 16.67 meters per second (60,000 meters divided by 3,600 seconds), this is the speed we will use for our calculations.
As Bridget observed, cars cover 50 meters in 3 seconds, which corroborates the speed of 16.67 m/s calculated from the speed limit. To cross safely, you need to be able to walk 4 meters before a car reaches you. We can use the speed to find the distance a car travels in the time it takes a person to walk 4 meters. If we assume an average walking speed of about 1.4 m/s, it would take approximately 2.86 seconds to walk 4 meters. Therefore, a car traveling at 16.67 m/s would cover about 47.60 meters in 2.86 seconds.
Dividing this distance by the length of an average car (3.5 meters), it is roughly 13.6 car-lengths that a car would travel while you're crossing. Therefore, a car should be more than 13.6 car-lengths away for it to be potentially safe to cross.