Deceleration Rate?

Why do we assume 11.2 ft/sec^2 for stopping sight distance, but 10 ft/sec^2 for clearance intervals?

John Q said:

Why do we assume 11.2 ft/sec^2 for stopping sight distance, but 10 ft/sec^2 for clearance intervals?

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Johnny-Q,

Remember this post? http://engineerboards.com/index.php?showtopic=24089&p=7223282

Well, the point is, each organization likely has their own recommended accel/decel rates. The rates are just averages based on field measurements. But the range of how people accel/decel is so great, even "averages" can have a big range. For instance, my sister keeps her foot on the gas all the way to the upcoming red light and then mashes the brake pedal right before the light (drives me nuts). If I see a red light ahead, I immediately let off the gas and start coasting toward the light. You see my point.

With that said, each organization should explain in their policy/manual/standard why they use the rate they do. For instance, I remember AASHTO (GDHS) explains the reason they use a decel = 11.2 ft/s2. I think they say it is the fastest stopping rate an average driver feels comfortable doing. I can't remember where the 10 ft/s2 comes from (is it the HCM? MUTCD?) but I suspect the book will state why they chose to use 10 ft/s2.

Also, keep in mind what the formulas these decel rates are being used for. SSD is the distance a driver needs to safely stop before hitting an

unanticipated object in the roadway. So you can expect a faster stop. In fact 11.2 is realistic but also conservative as a panic stop is more like 14.8 ft/s2. 10 ft/s2 for clearance might make sense because it is being more conservative and providing a longer yellow.

Thanks for the reply. I do remember that post.

I guess my real question, or observation is ... for the test....

Should I be using 11.2 ft/sec^2 for SSD and 10.0 ft/sec^2 for clearance?

Right now I am studying from review course notes and Principles of Highway Engineering and Traffic Analysis, both of which use the two separate deceleration rates for these problems.

John Q said:

Thanks for the reply. I do remember that post.

I guess my real question, or observation is ... for the test....

Should I be using 11.2 ft/sec^2 for SSD and 10.0 ft/sec^2 for clearance?

Right now I am studying from review course notes and Principles of Highway Engineering and Traffic Analysis, both of which use the two separate deceleration rates for these problems.

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Yeah, good question. 11.2 for anthing SSD for sure since that is right out of the GDHS. I'm sorry, I forget where the 10.0 comes from but I think originally the ITS Traffic Eng Handbook (which is no longer on NCEES's list). If it is industry standard to use a certain value for a given variable, you are safe to do so. But I'll comfort you in saying, unless it is 100% accepted by all (like gravity g = 32.2!), then the exam will almost certainly tell you what value to use.

ptatohed said:

But I'll comfort you in saying, unless it is 100% accepted by all (like gravity g = 32.2!), then the exam will almost certainly tell you what value to use.

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That's only an approximation for gravity on earth.

:borg: :borg: :borg: