October 16


This is a guest post from Bryant Ficek, PE, PTOE.  He’s our new vice-president at Spack Consulting and my co-author on the Traffic Study Manual.

An architect asked us to review a site plan he’s working on to see if we could support the access he’s proposing.  He’s showing a right-in/right-out on a divided collector road (city jurisdiction) midway between signalized intersections that are only 400 feet apart.

While this type of access clearly doesn’t meet standard access spacing guidelines, a right in access already existed on the other side of the main road and similar configurations exist in the city. Having done something in the past is never a good excuse for continuing to do something in the future. However, could some mitigating factors or characteristics allow us to support this proposed driveway?

The primary reason to follow the access spacing guidelines is for safety. Intersections (public and private) that are too close represent an increase in crash potential. Pedestrian and/or bicycle crashes are possible if drivers entering or exiting a driveway are too focused on traffic rather than other non-motorized vehicle traffic. Rear end crashes are more likely due to the confusion of which intersection or driveway the leading vehicle may be turning into. Angle crashes are another possibility when a motorist leaves a driveway expecting the approaching car on the main line to turn into the driveway. Finally, sideswipe crashes are a potential risk from cars leaving a driveway and cutting across multiple lanes to turn left at the downstream intersection.

Given the safety issues, our analysis needed to show that the proposed driveway would need to overcome the potential for increased crash risk through its design, the expected traffic operations, or some combination of the two.

The main road between the signals is a local city road providing two lanes of travel in each direction with turn lanes at the intersections. A raised median separates the traffic flow. The ADT is relatively low for a four lane road, under 10,000 vehicles per day. The main road didn’t have bike lanes or bus stops, although the proposed driveway would cross the sidewalk next to it. The proposed driveway would also intersect the main road in its right turn lane.

Watching the current traffic operations, the queues on the main line extended back to the upstream intersection a couple times in the morning peak hour and a few more times in the evening peak hour. However, off-peak times and other times during the peak periods, the queues were under 200 (about half the distance between the two driveways). We also noted that the downstream signal had an extremely long signal cycle length (200+ seconds) that was certainly contributing to the length of the queues.

After reviewing the existing site characteristics, we performed a micro-simulation analysis on the peak hours without and with the proposed development. Without the proposed development, the existing conditions matched our observations and the average and 95th percentile queues were less than 200 feet with one exception. The 95th percentile queue during the evening peak hour reached back to 300 feet.

Using standard trip generation, the proposed development would add about 70 vehicles in the morning and 135 vehicles in the evening. Re-running our analysis, the queues were not significantly affected by the proposed development and stayed about the same as existing. We also looked at the site if the long cycle length were reduced and found that queues did indeed decrease with this change.

Based on all this information, we came to the conclusion that a right out access could be allowed at this site. By reducing the access to right out only, the rear end and angle crash potential was eliminated. The proposed access could then also be located farther back from the downstream intersection, keeping the access out of the existing and future queues except during the very busiest times of the peak hours. An update of the signal timing would also improve the situation.

While this recommendation diverges from the standard guidance, it makes the best out of the facts as we understand them. It also best represents our desire to properly balance access and mobility on local roads.

  • I think I would check the following items as well:

    1. what is the turning radius for the proposed driveway ?
    2. what is width of the right hand lane, hopefully 15 foot or wider?
    3. what is length of the driveway throat? and see if entering vehicle can drive in without having slowdown too much that would block traffic on the collector.

    Just a thought.

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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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