Road Grades Don't Make Sense?

Question

I was driving through the Great Smoky Mountains yesterday, and I noticed road signs warning truckers of the steep grade. However, not a single one announced a grade of greater than 9%.

It felt like we were going down at about a 30 degree angle if not more. Even if you average out the grade over several miles, it still seems like it was much more severe than the signs claimed. It also doesn't seem like a 9% grade would be a very big deal to begin with.

I am well aware of how to calculate angles and slopes; what I'm asking is for people to discuss why the perceived steepness did not at all appear to match the signs.

Answer 1

The downgrade of the road is definetly different from the car driver to a truck driver. You wont feel the worry of your breaks loosing air in a car. Plus the wieght of the car is thousands of pounds lighter. The grades are there of course to warn the commercial driver of a steep grade approaching so they can get into the correct gear and adjust conditions to drive safetly. And hopefully they never have to use the runaway ramps..

Answer 2

A 9% grade means the road will drop 9 ft in 100 ft. A steep grade by anyones' standards.

Answer 3

hmmm....maybe they want to make some mad cash from insurance.

Answer 4

I seriously doubt that any public road in the US, except possibly some hills in San Fransisco are 30 degrees. They just plain wouldn't be safe to drive on. Remember that 45 degrees is half way to straight up, the way of measuring grades is different than measuring degrees in a circle.

Answer 5

I've never been able to figure out weather these signs mean 9 degrees or 9 percent. they use the percent sign but I think they mean degrees.

Answer 6

You are probably right with your conclusion that the signs don't match the grade.  

When the interstate highway system was designed,  it was determined that there would be no grades on the interstate system greater than 7%.  This sounds good, but it's not practical.  In Colorado, the signs on the westbound lanes of I-70 coming out of the Eisenhower Tunnels state 7% grade for six miles....  I would lay odds that the first half mile at the top is closer to 10% and then it may drop down to 7%.  

A little further west on the downhill side of the Westbound lanes of Vail Pass, the same thing happens.  It is marked 7%, and it closer to 9%.  

On secondary roads, the engineers tried to keep things at 9%, but like you have noticed, there are grades that are a lot steeper than 9%.  

My guess would be that in a "truck driver's" viewpoint, 9% is STEEP!!!!   Anything over that would just be "scare tactics".  

Having spent most of my adult life driving a truck and most of that in the Rockies, if you see a 9% marking, you are going to do everything in  your power to have the truck in the proper gear and under control before you drop off the hill.  

We have a street in Seattle that is marked 21% and all trucks are banned from being on that hill, either up or down....   Hard to imagine why, huh???

Hope this helps... Steep is steep.... and if the driver is prepared for it, it really doesn't matter how steep it is. (in my professional opinion, anyway)

Answer 7

The signs show the -%- sign,   meaning percent.    

So far,   Ken B has it.

"Many calculations used in accident reconstruction require a value for the grade or superelevation of a roadway.  To calculate this value, m, use the following formula:

                                                  m  =     vertical rise

                                                               horizontal run"

Expressed as a fraction,   divide the vertical rise by the run to get grade percentage.   Source is the first reference below.

More can be found at the second site below

As to the perception,  we are used to grades in our everyday life,  walking uphill or steps,  or driving them.   They are normal for us,  and this affects how we perceive them.    If you uses a 5% grade daily,  you will become accustomed to it and it's significance is lessened.    But when you come across a 20% grade,   you will still get the "whoa dude"   effect as you approach.