Q. I am writing about a subject that has perplexed me for a long time: namely, how DOT decides how long the interval should be for a light change. Frequently, I find myself in a long line of cars (not during rush hour) that moves slowly because a quick change from green back to red occurs. I’ve often wondered what the reason is that the lights are not set to remain green for longer intervals in order to allow more cars to pass and therefore, free up congestion. When I visit other states (such as Florida) I notice the lights there seem to stay green for several minutes.

A. RIDOT uses common engineering practices and accepted nationwide standards to determine traffic signal cycle lengths and how the time is split between the intersecting roads. We also take into consideration the traffic characteristics at the intersection such as traffic volumes, turning movements and the percentage of truck traffic.

A capacity analysis is done at the intersection using these characteristics to determine the overall delay (or idling) times and queue lengths for every approach to the intersection. The geometry of the intersection and surrounding road network characteristics are also considered when timing the signal.

For example, if there is a main road with many intersecting roads spaced evenly apart with equal volumes, we may coordinate the signals on the main road and give priority to moving this main thoroughfare’s traffic through first. In some cases an individual movement, such as a left turn or a side-street crossing, may be shortened in order for the overall operation of the signal to function at its best. It is the Department’s practice to consider the overall operation of the signal rather than individual movements. The priority, however, is typically given to the main road when timing a traffic signal.

Confusing 295 on-ramp

Q. I was traveling south on Route 44 in Smithfield this past weekend, intending to take I-295 North to Cumberland. The signs for Route 295 North and South were unclear to me. Can you clear up my confusion?

A. The location that you are referring to is the on-ramp for Route 295 South from Putnam Pike Eastbound. There is a traffic signal at the intersection of Putnam Pike and Smithfield Crossing, which is approximately 500 feet before the on-ramp for I-295 South. At this signalized intersection, there is an additional through lane added to the far right of the intersection. This lane continues through the intersection, and eventually becomes a right turn only lane for the on-ramp.

You are correct when you say that it is confusing and there is no indication that this lane becomes an exclusive right turn lane for the on-ramp. To clarify this we are going to install a lane control sign which will indicate that the two left lanes are for through traffic only, while the right lane is for right turns only.

We also plan to install a right arrow and the word “only” pavement markings in this right lane starting approximately 100 feet after the signalized intersection of Putnam Pike and Smithfield Crossing. This should make it clearer that the right lane is exclusively for the I-295 South on-ramp.

Dana Alexander Nolfe, chief public affairs officer for the state Department of Transportation, has agreed to answer questions of general interest posed by Journal readers about state roads and other state transportation matters.

The DOT is responsible for the state’s transportation infrastructure, which includes highways, bridges, traffic signals and bikeways.

To ask a question that would also be of interest to other readers, send a letter to Ask RIDOT, Features Department, The Providence Journal, 75 Fountain St., Providence, RI 02902. You can also e-mail your question to projocars@projo.com. Please put “Question for the DOT” in the subject field.

Questions or complaints of a specific nature should be posed to the DOT directly and will not be answered in this column.