Week 4 Flashcards
Safety Management Process
Network Screening –> Diagnosis –> Remedy –> Economic Appraisal –> Prioritize Project –> Evaluation
–> Network Screen (repeat)
What is Network Screen used to identify?
- hot spot
- hazard sites
- high risk sites
- accident prone sites
Network Screening Process
- establish focus
- identify network and establish reference population
- select performance measure
- screen and evaluate results
Network Screening Process - 1
ESTABLISH FOCUS
- objective is to evaluate a network to identify and rank sites
1) with a potential to reduce crash frequency or severity
2) with a particular crash type or severity
Network Screen Process - 2a
IDENTIFYING NETWORK
-identify the network elements that can be screened (intersections, roadway segments, ramps,…..)
Network Screen process 2b
ESTABLISHING REFERENCE POPULATION
- a reference population is a group of sites with similar characteristics
- prioritization of individual sites is made within a reference population
Network Screening Process 3
SELECT PERFORMANCE MEASURE
- select one or several performance measures to be used in evaluating the potential to reduce the number or severity of crashes at a site
- conventional measures: crash frequency, crash rate, crash severity, rate quality control technique
Network Screen Process 3 - Selecting performance measure: Variations in Crash Frequency. Long term vs short term?
- random events, naturally fluctuate
- difficult to determine if change in observed crash frequency is due to changes in site or natural fluctuations
- short term average frequency’s might be significantly higher/lower than long term average crash frequency
- short term will fluctuate, but will eventually go back to expected average crash frequency if its just random fluctuations youno?
Network Screen Process 3 - Selecting performance measure: Regression to the mean (RTM) in crashes.
- RTM is a tendancy for really high values to fall towards the average value and the equivalent tendancy for reallow low values to rise towards the average value in the same way
- thus, roads with a high number of crasehs in a particular period are likely to have fewer during the following period
- without accounting for RTM, a treatment could be considered successful even tho it wasnt
Network Screen Process 3 - Selecting performance measure: Variation in Roadway Characteristics and Environment
- Site characteristics are subject to change over time
- changes could be either Discrete(changing traffic control or road geometry) or Continual (traffic volume/weather)
- the variation of site conditions over time makes it difficult to attribute changes in the expected average crash frequency to specific conditions
Network Screen Process 3 - Selecting performance measure: List of Performance measures
- Crash frequency
- Crash Rate
- Crash Severity Method
- Rate Quality Control Technique
Network Screen Process 3 - Selecting performance measure: Crash Frequency
- uses observed collision counts to identify/evaluate safety
- compare to other similar entities, for example, when comparing intersections they all should be signalized, same number of legs, same range in AADT
- pretty much comparing collision counts between entities that have same setup to see which are safe or less safe
Network Screen Process 3 - Selecting performance measure: Crash Rate
- adjusts crash frequency using a measure of exposure
- must compare to similar entities (like crash frequency)
Network Screen Process 3 - Selecting performance measure: Pros and Cons of using Crash Rate
PROS
- easy to compute
- accounts for expure
- could be modifited to account for severity
CONS
- doesnt account for random variation of collisions
- prone to RTM bias
- low volume/crash bias
- invalid linearity assumption
Network Screen Process 3 - Selecting performance measure: Crash rate. Relationship?
Assuems linear relationship between # of crashes and AADT
- NOT THE FACT, actually, relationship between # of crashes and AADT is non linear
- for lower volumes, crash rate is very high
- as volumes go up, crash rate steadily goes down to a realistic rate
- this is called the low volume/crash bias
Network Screen Process 3 - Selecting performance measure: Crash Severity Method
Equivalent Property damage Only (EPDO) index
- assigns weighing factors to crashes by severity to develop a single comboned frequency and severity score per location
- greater importance, or weight, to collisions resulting in serious injury/fatality, followed by collisions resulting in moderate injury, and property damage only collisions
Network Screen Process 3 - Selecting performance measure: Crash severity Method - PROS / CONS
PROS
- easy to compute
- accounts for severity
- could be modified to account for exposure
CONS
- prone to RTM bias
- doesnt account for random variation of collisions
- highly influenced by the weighting factors used
Network Screen Process 3 - Selecting performance measure: Rate Quality Control Technique
- assumes collision rates are poisson distributed
- if the actual crash rate is greater than critical value, the deviation is probably not due to chance, but to the unfavorable characteristics of the intersection or road section
Network Screen Process 3 - Selecting performance measure: Rate Quality Control Technique PROS/CONS
PROS
- allows statistical testing
- uses threshhold for comparisons
- ACCOUNTS FOR RANDOM FLUCTUATION IN CRASH COUNTS
CONS
- prone to RTM bias
- invalid linearity assumption
Network Screen Process 4 = ???
Screen and evaluate results
Safety Management Process Part 1 = ??
Network Screening
-review network to identify and rank sites based on potential for crash reduction
Safety Management Process Part 2 = ??
Diagnosis
-evaluate historic crash data and field conditions to identify crash patterns
Diagnosis Process
- Safety Data Review
- Assess Supporting Documentation
- Assess Field Conditions
- Identify Concerns
Safety Management Process Part 3 =????
Remedy
-identify factors that may contribute to crashes at site and selecting possible countermeasures
Remedy Process
- Identifying Contributing Factors
2. Select Potential Countermeasures
Remedy Process - Crash modification factor CMF
-CMF represents the relative change in crash frequency due to a change in one specific condition
Remedy Process - CMF interpretation?
CMF > 1 –> increase in crashes
CMF = 1 —> no effect in crashes
CMF < 1 —> Reduction in crasehs
Remedy Process - CMF VS CRF
CRF = 1 - CMF
-for example, CMF of 0.9 implies a reduction factor 0.1 (expected percent change of 10%)
Remedy Process - CM Factors Vs CM Functions
a CMF is either
- multiplicative FACTOR used to compute the expected number of crashes after implementing a given countermeasure at a specific site
- a FORMULA used to compute the CMF for a specific site based on its characteristics
Remedy Process - CMF confidence interval
- if confidence interval doesnt include 1, CMF is significant at the given confidence interval
- if 1 falls within confidence interval, CMF is insignificant at that confidence level
Safety Management Process Part 4 =????
Economic Appraisal
-evaluate the benefits and costs of the possible countermeasure
Economic Appraisal steps (covered in chapter 13 week 12 so dont have to know probably)
- Quantify Crash reduction
- Convert Benefits to monetary values
- convert annual monetary value to present value
- estimate project costs
- conduct economic appraisal
Safety Management Process Part 5 =????
Prioritize Project
-evaluate economically justified improvements to meet objectives
Safety Management Process Part 6 =????
Evaluation
-evaluate the effectiveness of the implemented countermeasure
