Road Safety Analysis and Crash Reduction Strategies, Lecture notes of Engineering

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SAMPLE PROBLEMS

ON

ROAD SAFETY

CRASH FREQUENCY

The number of observed crashes per year.

CRASH RATE

The number of observed crashes per unit of traffic volume passing through the location.

COMPARING ROAD SEGMENTS BY CRASH FREQUENCY AND RATE ROAD SEGMENT A: A THREE-MILE SECTION OF ROAD THAT HAS HAD FOUR CRASHES OVER FIVE YEARS AND HAS A TRAFFIC VOLUME OF 4,000 VEHICLES PER DAY. ROAD SEGMENT B: A THREE-MILE SECTION OF ROAD THAT HAS HAD 10 CRASHES OVER FIVE YEARS AND HAS A TRAFFIC VOLUME OF 12, VEHICLES PER DAY. IF AN AGENCY IS COMPARING THESE SEGMENTS BASED ON CRASH FREQUENCY, THEY WOULD PRIORITIZE ROAD SEGMENT B FOR HAVING 10 CRASHES COMPARED TO ROAD SEGMENT A WHICH HAD FOUR CRASHES. IF COMPARING THESE SEGMENTS BASED ON CRASH RATE, THE AGENCY WOULD CALCULATE THE CRASH RATE OF ROAD SEGMENT A ACCORDING TO CRASH RATE, THE AGENCY WOULD PRIORITIZE ROAD SEGMENT A. THE PRIORITIZATION OF THESE TWO SEGMENTS CHANGES WHEN TRAFFIC VOLUME IS TAKEN INTO ACCOUNT.

PREDICTED CRASHES

The frequency of crashes per year that would be

predicted for a site based on the result of a crash

prediction model, called a safety performance

function (SPF).

CALCULATING BENEFITS DUE TO CRASH REDUCTION A CITY HAS A STOP-CONTROLLED INTERSECTION WITH AN EXPECTED CRASH FREQUENCY OF 10 CRASHES PER YEAR, CONSISTING OF ONE A-INJURY CRASH, ONE B-INJURY CRASH, TWO C-INJURY CRASHES, AND SIX PDO CRASHES. THE CITY IS CONSIDERING INSTALLING A ROUNDABOUT AT THE INTERSECTION. BASED ON A SEARCH OF THE FHWA CMF CLEARINGHOUSE, THEY DECIDE THAT THEY WILL USE A CMF OF 0.19 IN THE CALCULATION OF THE CRASH REDUCTION BENEFIT

1. 30 THIS CMF APPLIES ONLY TO SERIOUS AND MINOR INJURY CRASHES, SO THEY DO NOT USE IT TO ESTIMATE ANY REDUCTION TO FATAL OR PDO CRASHES (SEE NOTE). THEY MULTIPLY THE CMF BY THE EXPECTED CRASHES BEFORE ROUNDABOUT INSTALLATION TO DETERMINE THE EXPECTED CRASHES AFTER INSTALLATION: THUS, THE BENEFIT OF A ROUNDABOUT INSTALLATION IS EXPECTED TO BE A REDUCTION OF 0.81 A-INJURY CRASHES, 0.81 B- INJURY CRASHES, AND 1.62 C-INJURY CRASHES PER YEAR. NOTE: A ROUNDABOUT WOULD ALSO LIKELY BRING A REDUCTION TO FATAL AND PDO CRASHES (I.E., ADDITIONAL CMFS COULD BE INCORPORATED), BUT THE EXAMPLE HAS BEEN SIMPLIFIED TO A SINGLE CMF FOR ILLUSTRATION PURPOSES.

FOR EXAMPLE,

IF THE EXPECTED NUMBER OF CRASHES WITHOUT A

COUNTERMEASURE IS 5.6 CRASHES PER YEAR, AND THE CMF

FOR THE PARTICULAR COUNTERMEASURE IS 0.8, THEN THE

EXPECTED NUMBER OF CRASHES WITH THE

COUNTERMEASURE IS:

5.6 CRASHES PER YEAR X 0.8 = 4.48 CRASHES PER YEAR

TRANSPORTATION DEMAND ELASTICITY

When the elasticity is less than –1, the demand is described as being elastic, meaning that the resulting percentage change in quantity will be larger than the percentage change in price. In this case, demand is relatively sensitive to price change. However, when elasticity is between 0 and –1, the demand is described as being inelastic or relatively insensitive.

SAMPLE PROBLEM:

SAMPLE PROBLEM: