# Homework Help: Road Safety Calculus

1. May 16, 2013

### thebest99

Hi I am starting as a highway engineer. My work college has given me this to try any help woul dbe great.

A pedestrian requires to cross a two-way carriageway which is 9 metres wide to
access a railway station. Given that the road is subject to a 30mph speed limit,
wherein the V85% speed exceeds the limit by 24% and the pedestrian has good
physical and cognitive skills, calculate the maximum distance away from the crossingpoint that an approaching vehicle will need to be in order for the pedestrian to safely
complete the crossing movement. Present your methodology and the calculations

Calculating Safe Distances: s =u.t + ½ a.t²

v2 – u

s = v.t + s = -------------
2.µ.g

2. May 16, 2013

### tiny-tim

hi thebest99!

3. Jun 28, 2013

### thebest99

What i have worked out :

85th speed exceeds the limit by 24% = 37.mph.

travels at 16.63 m/s

average driver reaction time 1.5 seconds

16.63 x 9 = 149.67 m/s x 1.5 = 224.5

am i on the right wave lengths here, thanks for any help

4. Jun 28, 2013

### tiny-tim

no, you've multiplied the car's speed by the pedestrian's distance

speed times distance wouldn't give you anything anyway (the result would be in metres-squared per second!)

find the time it takes the pedestrian to cross, and then how far away the car must be not to reach the crossing in that time

5. Jun 28, 2013

### haruspex

There seems to be some information missing.
First, it's not clear whether the pedestrian is supposed to be getting across without the driver having to brake. If so, we need to know the pedestrian's speed. Conversely, we don't care about deceleration or thinking time.
If this is a pedestrian crossing, so the driver is supposed to stop, we need to know the car's deceleration, but we don't care about the width of the crossing.
Btw, it should be asking for the minimum distance, not the maximum. There is no upper limit.