Study Results: Right Turn on Red Percentages
In Minnesota, drivers are allowed to turn right at a signalized intersection when facing a red light after stopping and waiting for a safe gap in traffic (both cars and pedestrians). Other agencies may treat RTOR similarly or have various other laws like always prohibiting RTOR unless signed otherwise. Whatever the case, most drivers know their agency’s laws and operate their cars accordingly. No big deal.
In terms of traffic engineering and the analyses we complete, we wondered if RTOR was an important factor and, almost as important, what the appropriate factor or guideline should be when determining if the RTOR traffic matches existing conditions or future expectations. So, we set out to examine the RTOR factor and determine for ourselves its significance and how to guide our analysis decisions in the future.
Let’s be clear that this is a very niche, technical factor. Some software packages have developed their own formulas and complete the calculation for you. Our primary software is PTV’s Vistro, which follows the High Capacity Manual (HCM) exactly. Of course, since the HCM is silent on RTOR factors, Vistro leaves that factor to you. Completing a quick analysis of a standard signalized intersection showed the percentage of RTOR traffic can make a letter grade difference in results.
To complete this work, we collected data at 11 intersections (29 different approaches) and made observations. Lots and lots of somewhat boring, even-to-us-traffic-engineers, observations. Using the characteristics of each intersection, the results were graphed to determine if any correlation existing between the percentage of RTOR and a specific characteristic.
Cutting short a technical tale, we discovered very little correlation with most results all over the board. Instead, the closest indication of the percentage of RTOR traffic at a signalized intersection is the corresponding signal phase’s percentage of red time against the overall cycle length. Intuitively, this answer makes sense as the more red time faces an approach, the more opportunity right turning traffic has to complete the turn during the red.
While that roughly matched our typical procedure, we identified a typical range of RTOR from about 30% of the total right turning traffic up to 80%. Boiling this research down to the key take-away and next steps, we realized our approach had actually been under-representing the true number of RTOR traffic at any given intersection. In some cases, our default RTOR position was 20% or 25%, which is less the minimum value as we discovered.
Now, we have updated our approach and start with at least 30% of the right turning traffic occurs on red in the analysis. The percentage can be further refined through the specific characteristics of an intersection and our peak hour observations, such as high pedestrian traffic, sight distance constraints, or restricted movements. This research has made our software calibration better and provided us solid ground if challenged on this factor. While technical and boring to most, that’s important progress to improve our craft to us.