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ection i
max(ΔCOi) = maximum reduction value from all candidate intersections.
The conflict opportunities calculated in the event-based simulation program are used as a
safety surrogate measure. A left-turn conflict opportunity is assumed to occur when a left-turn
maneuver is made within 2 seconds of the critical gap period, whereas rear-end conflicts occur
whenever a vehicle joins a queue. Both left-turn and rear-end conflict opportunities are treated
equally. Detailed procedures on the calculation of conflict opportunities can be found in Ha and
Berg (1995).
RESULTS
Literature Review
Guidelines for Unsignalized Intersections
The first guidelines for unsignalized intersections were developed by Harmelink (1967),
and these guidelines are still used by traffic engineers in the field. Harmelink calculated the
probability of through vehicles being blocked by the left-turning vehicles using analytical
equations. He obtained the critical values of these probabilities (for different speeds) from a
survey of practicing engineers. He recommended the installation of left-turn lanes at volumes
where his analytical equations produced these probabilities. Harmelink probabilities, obtained
from the judgment of a panel of traffic engineers, are shown in Table 1. The guidelines obtained
by Harmelink are shown in Table 2. These guidelines are also available in the AASHTO Green
Book (2001).
Table 1. Probability Values Used in Harmelink Guidelines
Approach Speed (mph)
Design Operating Probability (ρ)
50 40 0.02
60 50 0.015
70 60 0.01
Source: Harmelink (1967).
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Table 2. AASHTO Guidelines for Left-turn Lanes on Two-Lane Highways
Advancing Volume
Opposing Volumes 5% Left Turns 10% Left Turns 20% Left Turns 30% Left Turns
40-mph operating speed
800 330 240 180 160
600 410 305 225 200
400 510 380 275 245
200 640 470 350 305
100 720 575 390 340
50-mph operating speed
800 280 210 165 135
600 350 260 195 170
400 430 320 240 210
200 550 400 300 270
100 615 445 335 295
60-mph operating speed
800 230 170 125 115
600 290 210 160 140
400 365 270 200 175
200 450 330 250 215
100 505 370 275 240
Source: AASHTO (2001).
One of the shortcomings of Harmelink’s guidelines, identified by Kikuchi and
Chakroborty (1991), is the use of residual gaps. For example, if there were four consecutive 6-
second gaps and the left-turn maneuver time was 4 seconds, the number of left-turn vehicles
served in Harmelink’s guidelines would be six instead of four. In reality, it should be four. This
is because the next vehicle to turn left could not use the residual 2 seconds. Other shortcomings,
identified by Kikuchi and Chakroborty (1991), are the definitions of arrival and departure rates.
In queuing theory, the unit of arrival and departure should be identical. However, Harmelink
used arrival rate on the basis of through vehicles and departure rate on the basis of left-turn
vehicles.
Kikuchi and Chakroborty (1991) modified the shortcomings of Harmelink’s guidelines
and revised the guidelines. The new guidelines are shown in Table 3; the revised guidelines
were not incorporated into AASHTO’s Green Book (2001). They further developed other
guidelines based on other criteria such as LOS and delay using a simulation program. However,
the