# So What...it's Only 5km/hr More

**Moderators:** hwybear, Radar Identified, Reflections, admin, Decatur, bend

- Reflections
- Moderator
**Posts:**1489**Joined:**Fri Apr 11, 2008 2:49 pm**Location:**somewhere in traffic

- Radar Identified
- Moderator
**Posts:**2881**Joined:**Mon Sep 08, 2008 8:26 pm**Location:**Toronto

Just did some physics here...

If a 1000-kg car goes at a speed of 108 km/hr (=30m/s - physics!) it will take approximately 51 metres for the vehicle to stop completely (that is assuming that the coefficient of friction between the tires and the road is 0.9 - pulled out of the physics textbook).

If the same vehicle travels at a speed of 113 km/hr (or 31.4 m/s) it will take roughly 56 metres to come to a complete stop. In 51 m of braking (ie same distance), the speed of the vehicle will be approximately 33.4 km/hr.

E=(m*v^2)/2=F*d=µ*m*g, F = µ*m*g,

1-st equation is kinetic energy equation, the second is work, the 3-rd is rearrangement of force of friction doing work, the 4-th is the force of friction equation.

where:

m - mass (in kg)

v - velocity (in m/s, to convert from km/hr divide by 3.6)

µ - friction coefficient, denoted by Greek letter mu (looks like u with a long tail on the left side of u).

g - acceleration due to gravity, 9.81 m/s^2

If you rearrange, the stopping distance d = m*v^2/(2*µ*m*g), or d=speed^2/17.658, since m cancels out, µ is constant (dry tires on dry pavement), g is also constant. So mass of the vehicle doesn't matter o_O, a truck will take as long....

On the same note, a vehicle travelling at 35 km/hr will take only 5.4 metres to stop (about 16 feet). What's going 40 when the stopping distance is 6.3 metres???

"The hardest thing to explain is the obvious"

www.OHTA.ca & www.OntarioHighwayTrafficAct.com

- Reflections
- Moderator
**Posts:**1489**Joined:**Fri Apr 11, 2008 2:49 pm**Location:**somewhere in traffic

Ya isn't that common sense though. So if driving 5 km/hr over is unsafe. Why don't we limit the highways to 50 km/h so the province can protect people from themselves?

People know this but they dont care. How do i know this. Cause when im on the 401 the guy in the left lane going 119 km/h with 10 cars on his bumper doesn't seem to care and nor do the people who are all trying to pass him.

Sure in the event of an accident lower speeds will do less harm. But what is to say they're will be an accident. What is to say that accidents and death have more to do with excessive speed as opposed to not paying attention. You can make things slower buts its hard to prove.

All we know is people drive these speeds everyday in many places around the world and they arent dying anymore or less than we do. Theyve done studies in the states and other European countries. They seemed to find the zones with higher or no limits didnt show they were any more dangerous then zones without speed limits.

The province never seems to understand cause and effect. With my example before. The province might decide to crack down on aggressive driving and tailgating or improper lane changes. So they cant spend a whole pile of our money trying to enforce laws. But they might fail to see the cause of the aggressive driving is because of improper lane usage that angers other drivers.

Sure you cant stop sooner but then why not limit all roads even lower? What is the best speed? Who knows how do we decide the right speed? Arent all traffic injuries speed related. The t-bone at a 50 km/hr zone at a city intersection has speed as a factor to.

Everyone drives to close. Just enter the 401 or the QEW in rush hour and Toronto. NO BODY travels 2 cars distance. Every car on the road is "tailgating." People don't seem to care.

Racer - think I understand the bottom line there....but what does not appear answered in that equation (please correct me if I am wrong) is the following:

But I guess I'm using the video as an example.....

if driver a = 108km and the other driver b = 113km/hr......and the transport blocks their path......driver b is obviously farther down the road, driver b reaction time will be the same, however the distance travelled during that reaction time is longer....., then the braking distance......so would the impact then not be higher than 33km/hr?

Here is a little typo diagram

A...108km........distance travelled........reaction time.....braking......stopped

B...113km.................distance travelled.............reaction time.....braking.......stopped

I guess what remains the same is once both start to brake, but distance travelled is what makes the difference.

ummm the video stipulates that they hit the brakes at the same time while driving neck-to-neck, though one is faster.

otherwise why not:

A........................108........dist.reaction........braking..........crash

B...113.........dist.reaction..........braking.......................stop

On a side note, none of the freeways we have with the speed limit of 90+ are not controlled access, so the situation is moot to begin with!

"The hardest thing to explain is the obvious"

www.OHTA.ca & www.OntarioHighwayTrafficAct.com

If the reaction time is approx 0.1 s, in that time the driver at 113 will travel exactly 139 mm (5.5 in) ahead of the 108 guy. If you take the reaction time to be 0.2 seconds, that's 278 mm (11 in)... When you are taking 50 m to stop, that difference in negligible, I also did round well more up when calculatin for 113 guy as opposed to 108 dude.

"The hardest thing to explain is the obvious"

www.OHTA.ca & www.OntarioHighwayTrafficAct.com

- Radar Identified
- Moderator
**Posts:**2881**Joined:**Mon Sep 08, 2008 8:26 pm**Location:**Toronto

Good work with the calculations, racer.

One more variable to add. Let's say a cargo van weighing, oh, say, 6000 pounds is travelling 60 km/h. What would his stopping distance be compared to a Civic/Corolla/Cavalier that weighs half as much, but is going 80 km/h?

Also... couple of other things to mention...

You have a 50% chance of being killed if you are rear-ended at 105 km/h, broadsided at 75 km/h, or hit head-on where both vehicles are doing about 45 km/h. That, of course, assumes that those speeds are attained at the moment of impact.

A pedestrian has a 90% chance of surviving if they are hit by a car at 32 km/h, but a 90% chance of dying if hit by a car at 65 km/h. Makes you wonder why residential side-streets have 50 km/h limits. Yeah, pedestrians shouldn't be in the middle of the road, but little kids will be more likely to chase after a soccer ball on a quiet sidestreet as opposed to a busy arterial throughfare.

Radar Identified wrote:Good work with the calculations, racer.

One more variable to add. Let's say a cargo van weighing, oh, say, 6000 pounds is travelling 60 km/h. What would his stopping distance be compared to a Civic/Corolla/Cavalier that weighs half as much, but is going 80 km/h?

Also... couple of other things to mention...

You have a 50% chance of being killed if you are rear-ended at 105 km/h, broadsided at 75 km/h, or hit head-on where both vehicles are doing about 45 km/h. That, of course, assumes that those speeds are attained at the moment of impact.

A pedestrian has a 90% chance of surviving if they are hit by a car at 32 km/h, but a 90% chance of dying if hit by a car at 65 km/h. Makes you wonder why residential side-streets have 50 km/h limits. Yeah, pedestrians shouldn't be in the middle of the road, but little kids will be more likely to chase after a soccer ball on a quiet sidestreet as opposed to a busy arterial throughfare.

Thank you, physics is my primary area of interest...

Physics tell us that a loaded cargo van will have a greater friction force acting on it due to its weight, therefore (providing the tires lock) it will take the vehicle the exact same distance to stop at it would for a smaller vehicle!

Here, m doesn't matter, ÃŽÂ¼ is known to be 0.9, g is constant at 9.81 m/s, and 2 is 2. So the only thing that affects your braking distance are your brakes and speed!

"The hardest thing to explain is the obvious"

www.OHTA.ca & www.OntarioHighwayTrafficAct.com

Radar Identified wrote:A pedestrian has a 90% chance of surviving if they are hit by a car at 32 km/h, but a 90% chance of dying if hit by a car at 65 km/h. Makes you wonder why residential side-streets have 50 km/h limits. Yeah, pedestrians shouldn't be in the middle of the road, but little kids will be more likely to chase after a soccer ball on a quiet sidestreet as opposed to a busy arterial throughfare.

Do you have the numbers for cyclists? I bike a lot (helps keeping the weight down with all the donuts), and I prefer to be a live cyclist to a dead one, thus I never go on the street with a posted limit of 60+.

"The hardest thing to explain is the obvious"

www.OHTA.ca & www.OntarioHighwayTrafficAct.com

### Who is online

Users browsing this forum: No registered users and 6 guests