Chapter 4' Spot speed, Travel time and Delay studies

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Chapter 4. Spot speed, Travel time and Delay
studies

• Spot speed and travel time are the most commonly
  used indicators of performance for traffic
  facilities and networks. Delays are often used
  to measure the performance of traffic flow at
  intersections.

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1. Spot speed

• Spot speed (????) is defined as the average speed
  of vehicles passing a point on a highway
• Spot speed belongs to TMS
• Spot speed is typically obtained under free flow
  conditions

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Central tendency of speed

• Mean speed (????)--- defined as average speed
• Mode speed (???)--- defined as the most frequent
  single value among observation
• Median speed (????)--- defined as the speed that
  equally divides the distribution of spot speeds
  meaning there are as many vehicles traveling at
  higher speeds as at lower speeds, or called 50th
  percentile speed

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Spot speed collection methods

• Stop-watch method (manual)
• Radar meters (hand-held or mounted
  on a vehicle)
• Loop detector method (????)
• Microwaved-detector (????)
• Video-frequency detector (????)

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Layout of loop detector
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Video frequency detector
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Common descriptive statistics

• Pace (?????)--- defined as a 10 miles per hour
  increment in speeds that encompassed the highest
  proportion of observed speeds
• Pace can be used to reflect safety situation
• The higher the percentage of vehicles traveling
  within the pace, the safer it is, and vise versa

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• Standard deviation (???)---
• Standard error (??)of the mean ---
• Range of true mean (????)of spot speed ---
• Required sample size (???)---
• Other parameters --- mode (??), median (???)

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• Standard deviation (???)---
• Standard error (??)of the mean ---
• Range of true mean (????)---

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Sample size requirement
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Application of spot speeds

• speed limit build-up
• trends establishment
• specific design applications
• specific control applications
• high-crash investigation

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2. Travel time studies

• In general travel time studies involve
  significant lengths of a facility or group of
  facilities forming a route. It is often
  conducted on the arterial (???)or major roadway,
  along which there are several intersections
  (signalized or stop/yield signs).

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Purpose of travel time studies

• To identify problem locations on facilities
• To determine level of service of the facility
• To provide some inputs for traffic assignment
  process
• To provide travel time for economic evaluation of
  transportation improvement
• To provide information for travelers

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Methods of travel time survey

• Technique for travel time studies ---- car
  following method(???)and license-plate
  method(???)GPS-based method
• Runs for travel time studies --- depend on the
  accuracy requirement, time and budget
• Travel time displays --- contour lines (???)

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?????????
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3. Delay studies

• Delay is defined as an extra time spent by
  drivers against their expectation
• Delay is often used to estimate economic losses
  --- more fuel consumption and low productivity
  of operation
• Delay is also a very important MOE (measure of
  effectiveness ????) in traffic engineering

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Types of Delay at Intersection
Time-Space Diagram
Distance
A
C
B
Time
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Basic types of delay

• Stopped-time delay (????) --- the time a vehicle
  spends stopped waiting to proceed through a
  signalized or STOP-controlled intersection
• Approach delay (???????) --- adds the delay due
  to deceleration to and acceleration from a stop
  to stopped time delay

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• Time-in-queue delay (????) --- the time between a
  vehicle joining the end of a queue at a
  signalized or STOP-controlled intersection and
  the time it crosses the STOP line to proceed
  through the intersection
• Control delay (????) --- the total delay at an
  intersection caused by a control device,
  including both time-in-queue delay plus delays
  due to acceleration and deceleration.

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Intersection delay studies

• Stopped-time delay --- difficult to measure in
  field
• Control delay --- time-in-queue delay plus
  deceleration/acceleration delay

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Uniform Delay
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Incremental Delay
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• Methods of intersection delay studies ---
• (1) license-plate (???)
• (2) sample-counting (????)
• (3) HCM 2000 cycle-based method (???)

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License-plate approach

• This approach is useful for particular study with
  small sample size
• This approach is relatively accurate
• Communications among surveyors play an important
  role in obtaining high match-up rate
• Computer programming is needed to handle huge
  match-up work

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Sample-counting approach

• It is used when queue length is not too long
• Number of lanes at approach is small
• The good point of this approach is its automatic
  adjustment
• The weak point of this approach is that average
  stopped delays can be obtained but their
  distribution can not be got
• Refer to Chapter 6 of textbook by Ren Fu Tian

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HCM 2000 cycle-based approach

• Similar to sample-counting method in collecting
  data but different in calculation
• Like sample-counting method this approach can
  get average control delay only, and cant come up
  with distribution of the delay

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