Purple Lights and Advanced Sign Strategies Cut Conflicts from Last Minute Lane Changing and Weaving by up to 43 Percent at a Toll Plaza in New Hampshire.

Before and After Analysis Performed to Investigate the Effect of Purple Light Strategies on Toll Plaza Lane Changing Movements.

Date Posted

Experimental Traffic Control Device Testing at New Hampshire Toll Plazas

Summary Information

The introduction of automated tolling to toll plazas created environments which require more guidance for drivers so that they can navigate them safely and efficiently. Toll plazas had been designed on a case-by-case basis which led to a lack of consistency and continuity which could confuse drivers. The goal of this project is to understand drivers’ intuitions and behaviors so that proper traffic control devices can be implemented. Purple lights (Figure 1) were deployed with advanced signage to improve operation efficiency and safety by reducing late weaving and lane changing. This specific site also had high-fog risks which increases the need for wayfinding lighting. The total study period stretched from October 10, 2006, to November 4, 2006.


The toll plaza is located on a freeway in a rural setting between two towns. There are six dedicated toll lanes in each direction with two flex lanes in the middle than adjust based upon time of day. Traditional traffic volume data was supplemented with video data that was manually processed to obtain lane changes. Four measures of effectiveness (MOE) were established: 

  • MOE 1: Lane changes by automated tolling customers
  • MOE 2: Lane changes by cash customers
  • MOE 3: Percent utilization of dedicated automated tolling lanes
  • MOE 4: Percent of toll violations.


  • MOE 1: A reduction in lane changes range from 8.5 to 31.2 percent was observed, but the number of lane changes from automated tolling customers was not statistically significant.
  • MOE 2: The cash customers performed significantly fewer lane changes with up to a 11 percent reduction for lane #3 (lane accepts both cash and automated tolling) and a 43 percent reduction for lane #6 (Exact Change lane).
  • MOE 3: A statistically significant shift in usage was observed from combined cash/automated tolling lanes into dedicated automated tolling lanes. The lane usage shift indicates that customers are now identifying and using the centrally located dedicated automatic tolling lanes instead of those on the right side of the toll plaza.
  • MOE4: A comparison of the “before” and “after-steady/solid” revealed a statistically significant reduction in toll violations in four out of seven lanes.
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