Module 2 Discussion B
Kishon
Module2BDiscussion.docxDiscussion: How Fast Should We Go?
Initial post due Thursday.
Imagine traveling from a gravel road onto a paved local road, then a state highway, and finally an interstate. Your destination stays the same, but the road conditions—and the speed limits—change. Why might a reasonable speed on one road be inappropriate on another? And does getting there faster justify using more gas?
For this discussion, connect motion, friction, reaction time, stopping distance, and fuel efficiency to the decisions communities make about speed limits.
Think About the Road
|
Road type |
Features to consider |
|
Gravel road |
Loose surface, traction, dust, curves, and visibility |
|
Paved local road |
Driveways, intersections, pedestrians, and nearby homes |
|
State highway |
Number of lanes, intersections, curves, and access to businesses |
|
Interstate highway |
Entrance and exit ramps, separated traffic, trucks, and traffic volume |
|
Other freeway |
Controlled access, separated traffic, urban congestion, and closely spaced exits |
These categories overlap: interstates are generally freeways, and some state highways are also freeways. Focus on the road’s features when considering an appropriate speed.
Compare Time and Fuel Use
Suppose you are taking a 100-mile trip. Compare traveling at a constant 65 mph with traveling at 75 mph, assuming no stops or traffic delays.
Use:
[ \text{Time (hours)}=\frac{\text{Distance (miles)}}{\text{Speed (mph)}} ]
Multiply hours by 60 to find minutes.
For fuel use, assume a gasoline-powered vehicle gets 30 MPG at 65 mph, and its fuel economy decreases by 15% at 75 mph. This is a classroom assumption; actual results depend on the vehicle and conditions.
[ \text{Estimated MPG at higher speed} =\text{MPG at lower speed}\times(1-r) ]
Here, (r=0.15), so:
[ 30(1-0.15)=25.5\text{ MPG} ]
Then calculate:
[ \text{Fuel used (gallons)}=\frac{\text{Distance (miles)}}{\text{Fuel economy (MPG)}} ]
[ \text{Fuel cost}=\text{Gallons used}\times\text{Price per gallon} ]
Assuming gasoline costs $3.00 per gallon, the comparison is:
|
Measurement |
At 65 mph |
At 75 mph |
|
Travel time |
92.3 minutes |
80.0 minutes |
|
Assumed fuel economy |
30.0 MPG |
25.5 MPG |
|
Fuel used |
3.33 gallons |
3.92 gallons |
|
Fuel cost |
$10.00 |
$11.76 |
In this example, the higher speed saves approximately 12.3 minutes, but uses 0.59 additional gallon and costs about $1.76 more.
FuelEconomy.gov explains that each vehicle has a different most-efficient speed, but gas mileage usually decreases rapidly above 50 mph.
Your Initial Post
Write approximately 350–450 words addressing these questions:
1. Why should limits differ by road? Compare gravel roads, paved local roads, state highways, and interstates or other freeways. Discuss how traction, visibility, intersections, and surrounding traffic affect safe motion. Include a specific road you know and describe its relevant features.
2. What changes when speed increases? Explain how higher speed affects reaction distance and braking distance. At the same reaction time, reaction distance increases directly with speed. In a simplified model with constant braking deceleration, braking distance increases with the square of speed. How should these relationships influence speed-limit decisions? Use a corvette in your response.
3. Is the time saved worth the fuel used? Use the 65-mph and 75-mph example to explain your position. How might your answer differ for an occasional traveler and someone making this trip every workday? Explain how air resistance contributes to fuel use at higher speeds and identify one limitation of the example.
4. How do other countries approach speed limits? Use the official resources below to compare passenger-car motorway limits in Great Britain and France. Identify the units and any relevant weather or driver restrictions. Convert one French limit to mph using 1 mile ≈ 1.609 kilometers. Would either approach work well in the United States? Explain why road design and conditions matter when making that comparison.
5. What limit would you recommend for Mississippi interstates? Choose a specific maximum speed in mph for passenger cars under clear, dry conditions. Support your recommendation with physics, the time-and-fuel comparison, and the international examples. Explain one benefit and one drawback. Would you recommend different limits for rural stretches, busy urban sections, or poor weather?
Your recommendation is a proposal, not a statement of the current legal limit. A posted maximum also does not guarantee that driving at that speed is safe under all conditions.
Resources
· Great Britain: Official speed limitsLinks to an external site.
· France: Official speed limits and weather restrictionsLinks to an external site.
· NHTSA: Speeding and its consequencesLinks to an external site.
· FuelEconomy.gov: Driving more efficientlyLinks to an external site.
