August 30


I've been called in to help a developer on a 370 unit apartment project proposed on a vacant parcel.  We're still working on this, so I'm going to keep the discussion somewhat generic.  The apartment building's parking ramp will have a driveway on both the north and south side of the parcel.  I'm going to focus on the south side driveway, where there's a potential issue.

The parcel is on the northwest corner of a north-south major collector and an east-west minor collector (city's designations), with traffic largely destined to the south on the major collector as that's a quarter mile from a freeway interchange and a large shopping district south of the interchange.  The east-west minor collector carries about 5,000 vehicles per day. The city's 20 year forecast bumps this up to about 7,000 vehicles per day, but this apartment building largely accounts for that growth. 

Immediately south of the minor collector is a large corporate headquarters with its parking ramp accessed via the minor collector.  To complicate things, there's a railroad line running along the western edge of the development parcel.

The city is trying to promote development in this area and they've had a couple of deals fall through on this parcel.  They also studied traffic in the area and have upgrades planned to the major collector/minor collector intersection on the development corner.

The developer is a national apartment developer with about 12,000 units under their belt.  Not their first rodeo – they know what they need in order for the deal to work.  Their parking ramp is next to the railroad tracks to buffer the apartment units from railroad noise and also to provide the number of units they need to make the project viable.  This puts their driveway about 130 feet west of the primary parking ramp driveway to the corporate headquarters building – setting up offset tee intersections.

The developer raised this design as critical in an initial meeting with the city.  City staff indicated they were ok with the offset driveway design.  The city has had their consultant traffic engineer study the north-south major collector – an update to their previous work to confirm the upgrades are still appropriate.  They didn't analyze the driveways on the east-west minor collector.  Presumably because they were deemed to be acceptable.

Off-setting, tee intersections aren't a great idea on minor arterials or even major collectors. Access management (limiting driveway access points) has well proven safety benefits.  Access management on local streets and minor collectors is less of a concern.

The city engineer just raised the offsetting tee driveway on the minor collector as an issue and has said he will recommend aligning the driveway with the corporate headquarters driveway 130 feet to the east.  The developer is very upset since this is truly a deal breaker.  They won't even bring the development before the planning commission if moving the driveway (and redesigning the site) is a staff condition.

I'm attending a meeting next week with city staff to try to work this out.  Below is a diagram of the situation and here are reasons why I think the proposed offsetting driveway design is acceptable:

  • This is a weekday, a.m. peak hour issue only (3 to 6 hours out of the whole week).  The apartment traffic and corporate headquarters won't cross paths in the p.m. since the vast amount of traffic from the apartment as well as the corporate headquarter's traffic are oriented to the east.
  • Capacity analyses were not done on the driveways in the city traffic study, but I'm confident the proposed design has little delay at each driveway intersection.  Moving the proposed driveway to line up with the corporate headquarter's would actually cause significantly more delay for the outbound apartment traffic in the morning because those 55 vehicles would be turning against 625 vehicles.  The proposed design has them turning against approximately 250 vehicles.  With the proposed design, the 55 apartment vehicles would conflict with the 350 left turning corporate headquarters traffic but that will not move the needle on the delay calculations.
  • It is likely the apartment traffic will leave in the morning before the work traffic shows up at the corporate headquarters building, minimizing conflicts.
  • The apartment traffic can go out the northern driveway, even though it is less convenient.  If this design proves to be problematic, those daily commuters will shift their pattern so it's not a problem.


  • Delay, delay, delay. Everyone’s worried about delay.
    But offset tees, if they have enough offset, usually are safer than one four-way intersection.
    Is there enough clearance between the tracks and the apartment driveway?

  • Strange twist – we ended up moving the driveway to line up with the driveway right next to the tracks. There was a big communication breakdown between the site designer and city staff that we brokered in a meeting.
    This change was workable for the developer. I personally prefer the offset tee instead of the driveway being so close to the railroad track. We’re in the gray area of professional judgement though and I didn’t argue since I was hired to help make the development happen. Given the big picture, I think this situation will work too and we’re not introducing a significant safety hazard.

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    Mike Spack

    My mission is to help traffic engineers, transportation planners, and other transportation professionals improve our world.

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