CE 361 Introduction to Transportation Engineering



|CE 361 Introduction to Transportation Engineering |Posted: Sun. 19 October 2008 |

|Homework 7 (HW 7) Solutions |Due: Mon. 27 October 2008 |

VEHICLE AND DRIVER CHARACTERISTICS

□ Work alone or as part of a group of up to three CE361 students. Group members must sign the front page of the work submitted.

□ Label each problem with its name.

Failure to comply with these requirements will result in penalties.

1. Roundabouts and Safety. In HW2, two solutions to the traffic safety problems at Renner Blvd and Dell Avenue were evaluated. A third solution is possible: a roundabout. A recent national study found that the installation of roundabouts led to a 45-percent decrease in total crash rates, a 79-percent reduction in injury crash rates, a 95-percent decrease in the fatal crash rate, and a 27-percent reduction in property-damage-only (PDO) crash rates. During the last five years, the intersection of Renner and Dell has had 2 fatal crashes, 46 personal injury crashes, and 397 PDO crashes. The four approaches to the

intersection have ADTs of 11,908, 11,994, 8892, and 12,473.

A. (10 points) Calculate the RMEV for total crashes and for each crash severity.

(6.1) RMEV total = [pic]*106 = [pic]= 10.77

(6.1) RMEV fatal = [pic]*106 = [pic]= 0.048

(6.1) RMEV PI= [pic]*106 = [pic]= 1.114

(6.1) RMEV PDO= [pic]*106 = [pic]= 9.61

B. (15 points) The travel demand modelers in Mythaca estimate the approach volumes at Renner and Dell will be 17,793, 19,477, 10,767, and 15,714 in Year 2025. If the changes in crash rates due to roundabouts from the national study apply to this intersection, estimate the number of total crashes and crashes of each severity that will occur at Renner and Dell in Year 2025. Any comments?

New (Year 2025) Approach volume = 0.5*(17,793+19,477+10,767+15,714)*365 = 11,634,558.

New Total RMEV = (1-0.45) * 10.77 = 5.92.

New total crashes = 5.92*11.634 = 68.9, down from 89 per year.

New Fatal RMEV = (1-0.95) * 0.048 = 0.0024.

New Fatal crashes = 0.0024*11.634 = 0.028, down from 0.4 per year.

New PI RMEV = (1-0.79) * 1.114= 0.234.

New PI crashes = 0.234*11.634 = 2.72, down from 9.2 per year.

New PDO RMEV = (1-0.27) * 9.61= 7.015.

New PDO crashes = 7.015*11.634 = 81.62, down from 79.4 per year.

Comments: (a) Even with greater entering volumes, the number of crashes with all severities will decrease. (b) Least decrease is with PDO crashes. (c) Sum of predicted Fatal+PI+PDO crashes in Year 2025 = 0.028+2.72+81.62 =84.37, which does not equal predicted Year 2025 total of 68.9 crashes. In fact, RMEV PDO > RMEV total! Why?

1. Reading the message. The 85th percentile speed of westbound drivers at mile marker 22 on SR361 is 68.3 mph. Five hundred feet beyond the mile marker, a flagger at a workzone lane closure may need to stop WB traffic to allow traffic in the opposing direction to go eastbound. The highway department wants to rent a dynamic message sign (see figure at right) to warn WB drivers of the situation.

A. (10 points) Calculate the distance it will take the “design driver” to stop after seeing the DMS. How did you determine that distance?

In FTE Table 6.11, 68.3mph is closer to 70mph than 65 mph, so use f=0.28 in (6.17). SSD= d(rcn)+d(br) = (trcn*vo)+[pic] = (2.5*68.3*1.47)+ [pic] = 251.0 ft + 559.0 ft = 810.0 ft. This falls between 724ft (65mph) and 840ft (70mph) in Table 6.11.

B. (5 points) If the DMS needs to be placed at mile marker 22, how far before the mile marker should a driver be able to first see the DMS?

810.0ft-500ft=310.0ft

C. (10 points) The cost to rent a DMS depends on the character height. The DMSs available for rent have character heights of 13, 12, 10, and 9 inches. What is the smallest available character height that will allow drivers to stop before the flagger location? Let us say that the “design driver” has a Visual Angle value 1.5 times the mean value found by Smith (1979).

In FTE Figure 6.18, Smith’s mean is 0.0019 radians.

0.0019*1.5=0.00285. In FTE Figure 6.17, H=D*[pic]=310.0ft*0.00285=0.8835ft=10.6in.

The 10-inch characters will not be large enough. Use 12-inch characters.

2. (20 points) Skidding to a stop. Redo FTE Example 6.14, this time with fskid=0.1. What values of initial velocity vo and apparent response time tr do you get? Does this change the conclusions about whether the driver reacted to either sign and was exceeding the speed limit?

(6.16)’ vo2 = 2gfdskid + vf2 = (25*1.47)2 + 2(32.2)(0.10)(300)

vo2 = 1350.56 + 1932 = 3282.56; vo= 57.3 fps = 38.97 mph. This is well below the 55-mph speed limit, so the driver was not speeding at the time the skid began.

The driver’s reaction time to the second warning sign, if she drove at a constant 57.3fps before starting to skid, would have been t(rcn)=[pic]= 5.24 sec. This is much greater than the AASHTO standard of 2.5 seconds. The most likely situation is that the driver slowed down after the second sign, then jammed on the brakes 300 ft from the barrier.

3. Traffic control devices. Are stops signs a good way to controls speeds in residential neighborhoods? Do a brief search in the library or on the Internet and find at least three credible sources on the subject.

A. (15 points) List the references so that someone else could find them. What are their positions on the topic?

I entered “stop sign speed control” into and found: the following sites.

(1)

The City of Pleasanton, California. Each year, The City receives many inquiries about installing stop signs as a way to reduce speeding. However, research shows that other measures are more effective than adding stop signs. The purpose of stop signs is to assign right-of-way at an intersection, not to control speed. Reasons why stop signs are not a good speed control tool:

□ Drivers are forced to come to a complete stop, even if they are going the speed limit

□ Potential increase in rear end collisions

□ Drivers tend to increase speeds between stops

□ Increased noise due to hard starts and stops

(2)

W. Martin Bretherton Jr., “Multi-way Stops - The Research Shows the MUTCD is Correct!” Reviewed over 70 technical papers. The research found that, overwhelmingly, multi-way stop signs do NOT control speed except under very limited conditions.

(3)

“STOP SIGNS AND ALL-WAY STOP CONTROL“. Studies show that a stop sign will only result in lower vehicle operating speeds approximately 50 -100 metres (about 3-or 4 houses) before and after the intersection. Before and after studies of installing all-way stop control indicate that vehicle operating speeds between stop controlled intersections (mid-block) increase as much as 10 km/h

higher than before the stop signs were installed, as motorists try to make up for the delay. This higher speed increases the risk to pedestrians and other motorists and increases the risk of collisions.

B. (5 points) What is your conclusion?

Stop signs do not reduce speeds on residential streets. In fact, they may cause other problems, such as increased noise and vehicle exhaust emissions.

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