You Too Can Undo the Skew!
By: Timothy Miller, P.E.

Many of the traffic flow problems facing today’s City Traffic Engineers are "inherited" from the time the streets were platted, often over a century ago. Street grids and alignments that served horse drawn traffic and early motor vehicles well often bulge at their seams under today’s intense traffic loads. One typical problem is that of skewed intersections where diagonal roads cut through the normal street grid.

Steve Mullen, City Traffic Engineer for the City of Auburn, Washington was in such a position as he considered what could be done to ease congestion on a heavily traveled arterial street, Harvey Road. Although this section Harvey road had been widened to five lanes in recent years, heavy SB left turning traffic at the signalized intersection of 8th Street NE led the designers to specify L/TL/TR channelization for the over 900 left turning vehicles. 8th Street is one of the few streets that span the Green River. This street then climbs up the steep east valley hill to serve growing residential areas in unincorporated King County. Indeed the next similar major route to the north is over 4 miles distant, and to the south is one mile away.

The Splits and the Vanishing Point
As always the TL option lane forced split phase operation of the signal. Consequently the signal control was designed to allow Harvey Road to flow NB, then SB, but never concurrently. As you can imagine, the resultant traffic queues and vehicle delay was substantial.

• Two other factors affected the design and operation of this intersection.
• The intersection was skewed, (how did you guess?) and...
• The narrowing of Harvey Road south of the intersection.

The angle and orientation of the skew created the situation where the NB and SB left turn vehicle paths overlapped making simultaneous LT movements impossible. The narrowing of Harvey Road from five lanes down to two lanes immediately south of the intersection created what Hal Owens, Auburn’s Traffic Signal Technician once referred to as the "vanishing point" where two southbound lanes abruptly merge into one. The resultant turbulence in the SB traffic flow caused by this merge greatly impaired the effectiveness of having two SB lanes at the intersection.

Growing Pains & Alka Seltzer
While area traffic continued to grow at a steady rate due to strong regional economic health, Auburn itself experienced a development "boom". The Supermall of the Great Northwest (super-regional off price mall), Emerald Downs (thoroughbred racing track) and the Muckleshoot Casino all were constructed in Auburn over the span of only three years.

In order to tackle some of the traffic problems that were not being directly mitigated by developments, Steve Mullen secured added funding to his budget. This funding was used to provide on-call supplemental traffic engineering consulting services to the city. The firm of Transportation Consulting Northwest (TCN) was selected and Steve assigned solving the problems at the intersection of Harvey Road and 8th Street NE as the first task.

Ponderables Pondered
TCN immediately began investigating the intersection geometrics in detail using truck turning templates. We verified the skew angle eliminated the possibility of simultaneous NB & SB left turn movements. We demonstrated any geometric improvements to eliminate the conflict would have required major reconstruction and ROW acquisition. Since this was clearly beyond the intended scope of the improvements TCN then performed a series of level of service (LOS) runs using the Highway Capacity Software (HCS) see if other operational methods could be applied. We found that greatly improved intersection operation would result from converting the SB approach to L/L/TR and using lead-lag left turn phasing.

Another Time or Another Place
Since the NB and SB left turn vehicle paths conflicted (which would have resulted in head-on collisions), Our design separated these conflicting movements in time, rather than physical space. (Sounds like science fiction, doesn’t it?). The lead-lag left turn phasing operation was implemented so that NB left turn phase would lead (i.e. go first) along with it’s associated NB through phase. Next, the NB left turn phase is terminated and the NB though phase continues while the SB through phase starts up. After the NB and SB through phases have operated concurrently for a period, the NB through phase is terminated and the SB (lagging) left turn phase starts up. Finally the SB through and SB left turn phases terminate at the same time and the side street phases begin. The key to this improvement is the fact that finally the NB and SB phases can operate at the same time. No more through traffic stopped, waiting when there are no opposing left turns needing to be serviced.

Incredible "Invisible" Buttons
One unusual aspect of this project was related to the overlapping of the left turn vehicle paths. To enforce the double SB left turn movements, pavement lane lines separating the two left turn lanes and forcing the formerly option lane to now turn left were added. A complication to this was that northbound traffic could easily be confused by the button pattern that would be in their path. To eliminate this problem, TCN specified white/black dual color buttons be used. The white portion faced SB traffic while the black portion faced NB traffic, effectively eliminating them from distracting NB drivers. Where do you get such buttons? You don’t. Mike Lilstrom of Apply-A-Line manufactures them using an ancient Chinese secret process (OK, he uses a saw) to cut black and white buttons in half and then applies them separately. So it’s not rocket science -- but it works!

Big Bang (for the Buck) & The Force
To keep the design costs low, TCN made base maps from two of the previous street widening plan sheets. One had new signing and striping details added and the other sheet had signal wiring and phasing design & construction details added.

To keep the construction costs low the City then used TCN’s plans as part of a large work order to have signing and signal changes done by City forces. New pavement markings were installed using a contractor, Apply-A-Line.

Econolite Control Products of Anaheim, CA furnished a specially modified PROM chip to be installed in the existing ASC/2 controller to implement the lead-lag phasing. City signal crews modified the detection wiring and fire preemption systems accordingly. The conversion was accomplished over a three day period with City crews and the pavement marking contractor working in close cooperation.

Conclusions
Signalized intersections come in many flavors. Some flavors (like split phasing) can have serious drawbacks. Others (like skew) are not possible to eliminate. But with careful consideration and innovative application of various traffic engineering techniques, cost effective solutions can be found that will please your citizens, City Councilmembers and your boss!