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ir simulation program has assumptions that are not realistic, e.g., the use of a fixed 6-second
critical gap for left-turn vehicles, which may be too large for some situations. Kikuchi and
Chakroborty (1991) also provided a few new guidelines based on a LOS A/B cut-off for the
through vehicles and an arbitrary delay saving value of 14 seconds.
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Table 3. Volume Combinations Justifying a Left-turn Lane under Modified Harmelink Guidelines
Advancing Volume (vph)
Opposing Volumes
(vph) 5% Left Turns 10% Left Turns 20% Left Turns 30% Left Turns
40 mph operating speed
800 434 300 219 189
600 542 375 272 234
400 682 472 343 293
200 863 600 435 375
100 946 679 493 424
50 mph operating speed
800 366 257 185 162
600 460 320 234 202
400 577 403 294 255
200 735 513 373 324
100 830 576 424 365
60 mph operating speed
800 294 207 154 146
600 365 259 187 165
400 461 324 238 206
200 586 414 303 263
100 663 468 344 297
Source: Kikuchi and Chakroborty (1991).
Guidelines for Signalized Intersections
There are no commonly accepted guidelines on when to provide a left-turn lane at a
signalized intersection. Oppenlander and Bianchi (1990) proposed one set of left-turn lane
guidelines for signalized intersections. They suggested two criteria and recommended lanes
whenever intersections met at least one of two criteria. The first was the capacity requirement
based on the methodology for signalized intersections in the Highway Capacity Manual (HCM)
(1985); the second was the storage requirement for storing at least one left-turning vehicle. The
storage requirement was modeled using a queuing theory assuming Poisson arrivals and
exponential services. The design left-turn volume was used for arrival rate, and a function of
permitted left-turn capacity and opposing traffic was used for service rate.

Conflict Opportunities
The safety of an intersection is usually examined by an examination of its crash history.
However, crashes are rare events and no crash history is available for new developments. Thus,
surrogates for crash history can be used. Conflict analysis is one type of safety surrogate that is
often used. Conflicts can be defined as events such as near misses or sudden braking of the
vehicles that were about to collide. Unfortunately, conflict data also must be observed in the
field, which means that conflict analysis cannot be used for intersections that have not been
constructed.
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Ha and Berg (1995) defined another type of safety surrogate as conflict opportunities. A
conflict opportunity occurs when there is a potential for a collision between two vehicles. There
are two main types of conflict opportunities: left-turning conflicts and rear-end conflicts. If an
opposing vehicle is too close to the left-turning vehicle as the vehicle makes the turn, the event
could be treated as a left-turn conflict opportunity. Similarly, there is an opportunity for a rearend
collision to occur whenever every single vehicle joins the queue. Therefore every vehicle
joining the queue could be treated as a rear-end conflict opportunity. An advantage of conflict
opportunities is that they can be generated theoretically based on volumes, traffic control, and
intersection geometry. This means that, unlike conflicts, conflict opportunities can be estimated