Curves on the road

Factual Questions
1

Here’s something that’s been puzzling me lately:

Let’s say you’re driving on a two-lane highway at a fast speed. When the road curves to the right, it’s easier to maintain control of the car and maintain speed (no screeching of tires) if you’re in the right lane - the “inner” part of the curve. The same phenomenon, of course, is true on the left side as well.

But why should this be? The two lanes both represent the same degree of arc, but the distance traversed in the inner part of the curve is shorter than that in the outer part. So logically, you should need a sharper angle of your tires to drive that inner lane. Shouldn’t a sharper curve result in less control of the car?

What am I not understanding here?

2

If you are entering the curves in the outside lane and cutting down into the inner lane (breifly) at the center of the curve, then drifting back into the outer lane as you pull out of the curve, you are doing it right, and it is easiest way to maintain both speed and control.

Have no idea of the physics involved, though

3

You state your observations of which lane is easier to drive in as if they are fact. I haven’t noticed the phenomena you’re talking about. Are you sure it’s not imaginary? Are you perhaps going slower when you’re on the inside lane?

4

One word: camber[sup]1[/sup]

The road surface isn’t flat. In fact the road is curved upwards in the middle to aid drainage. When you take the innermost lane through the curve, the camber helps to keep you ‘in’, whereas in the outer lane it gives rise to an outwards force.
[sup]1[/sup]Note that camber can also mean the inclination of the wheels on a car. A source of endless confusion…

5

Popup, not so sure about camber.

if you are on a road where you will be going fast, the inside of the turn (the right side of the road, in your example) is lower than the outside (left side).

on a slow road, the grade will not be noticable (and perhaps not present), so camber could come into play. but ** cmkeller** said he was going fast, and i doubt camber would affect the turning that much at higher speeds.

jb

6

It might just be a matter of perception, cmkeller.

When you’re taking the inside lane through a corner, if you’re going too fast or have misjudged your line through the corner, you can drift outward a bit and there’s another lane there. As long as there’s no one coming the other direction, you’re fine. In the outside lane there’s less margin for error. If you drift a little too far, you’re into the guard rail or a ditch. What you’re noticing may just be a little apprehension at that fact.

7

What we’re attempting to discuss here is called ‘superelevation,’ in highway construction parlance. The art/science of designing highways includes methods of making curves seem like they’re not. One way of doing this is by ‘superelevating;’ other ways are less spectacular – reducing speeds, for example.

Superelevation is a way of constructing a road so that a turn (a horizontal curve) is less perceptible than it would be if the road maintained its ‘normal’ configuration. An easy way of demonstrating superelevation is by trying to ‘bend’ a straight, narrow strip of paper into a curve. The ‘outside’ edge of the paper will move upward relative to the ‘inside’ edge.

Go ahead! Try it! It’s fun! Slice yourself a 1" wide strip off the long side of a piece of copy paper. Grasp the ends of the strip and try to ‘bend’ it horizontally, as to represent a curve in a road. You’ll see the edge away from the direction of your ‘bend’ either raise or lower relative to the edge toward the ‘bend.’

NASCAR fans know, probably without understanding why, that a ‘high-banked’ track makes for a faster race. Superelevation is the reason.

What you’re doing in concept is twisting the plane of the road so that the actual width and length are the same, but the angles relative to level and direction of travel change. I can build a superelevated curve with the same amount of asphalt or concrete as it would take to build a level road of the same length, if measured on the centerline.

Here in Ohio, the standard cross-slope for a ‘normal’ section of roadway – two-lane, that is – is 3/16" per foot fall from the centerline (approximately 1.56%). This ‘normal’ profile holds as long as the horizontal curve (left or right) is not too extreme. Such a design allows rainwater to shed off the road in both directions from the centerline with reasonable dispatch, while at the same time allowing you to tool down same road without leaning too much on the steering wheel. (NOTE: When you have to lean a bit, it’s almost always to the left, eh?) This standard holds true for divided highways also, with some exceptions; you’ll be drifting toward the shoulder, either right or left.

BUT, when the road has to change direction sharply, we enter the world of superelevated curves. Suddenly, we have desk-bound engineering types twisting little strips of paper and trying to figure out how sharply to angle the road. Most of the time, I’m happy to report, they’re close in their estimation.

I’ve worked on Interstates that were superelevated as much as 3/4" per foot, about 6.2%. Allowing for the original negative slope of roughly 1.6%, that is a slope change of nearly 8 percent.

I’m not trying to mire this discussion in numbers. (Really, I’m not!) Let’s just say that without superelevating many of our roadways, we’d all be spending a lot more time in ditches, looking up at life passing us by.

Referring (FINALLY!!!) to the OP, on a standard two-lane highway, the right-hand curves are easier in the right (in this case, correct) lane and the left-hand curves are easier in the left (in this case, WRONG!!!) lane. And now, maybe, you know why.

8

The curves are banked. The forces excerted on the car by the banking depend on the angle of the bank, the radius of the curve, and the speed of the vehicle. Banked Curves The changing radius of the curve from the inside to the outside affects your ability to control the car. You can change the “steering neutral” path through a curve by speeding up or slowing down.