Two letters just in raise questions about a subject that seldom turns up in my mail: the arrows one occasionally sees on traffic signals.

A letter from Hannah Wexler asked. "Why doesn't Washington have any green left-turn arrows?" It is difficult to make left turns at many intersections, Hannah points out, and arrows would be a great convenience.

A spokesman in the District's traffic engineering office said we may not have many, but we do have "several" - roughly 25. "In some locations," he said, "we find them extremely useful." But the District hasn't been quite as quick to switch to arrows as some nearby jurisdictions, notably the state of Maryland.

Why don't we have left-turn arrows at more intersections? Because every time an "exclusive" or "protected" movement is provided, it takes the signal more time to go through one complete light cycle.

Example: If you provide 30 seconds of green light for East-West traffic and 30 seconds of green light for North-South traffic, and 5 seconds of yellow after every green, one complete light cycle will require 70 seconds.

If a "Walk" interval of 20 seconds is added, the cycle takes 90 seconds to run its course. If a 15-second arrow is added to give left-turners an exclusive or protected period for turning, the cycle's duration grows to 105 seconds.

At that point, straight-ahead traffic in both directions is seriously impeded. Instead of moving during 30 seconds out of each 70-second cycle, it now moves for only 30 seconds during each 105-second cycle. It is at a standstill for 2 1/2 times that long - 75 seconds.

You don't have to be a traffic engineer to see that arrows provide a useful service but cause delays that engineers abhor. Arrows must be used with discretion.

The second letter about traffic arrows came from Prof. Azriel Rosenfeld, who wrote: "I have been told that when there is a green light (not an arrow) at an intersection, it is legal to make a left turn even if there is a left-turn signal and it is red. Can you get someone to confirm this?"

No, I cannot. There may be a jurisdiction somewhere that permits such a turn, but it would not be in conformity with the uniform code manual that traffic engineers depend upon to make traffic control devices intelligible to tourists from all 50 states.

What follows is a summary of what I was told by traffic authorities in this area:

In the absence of restrictions to the contrary, a full green light gives you permission to go straight ahead or turn to the left or right.

A "restriction to the contrary", might consist of a sign forbidding a right or left turn, a sign making a turn mandatory, or a red traffic arrow.

Note the difference between the old style arrows, which appear only when they are green, and the newer signals that display arrows in changing colors.

At the old signals, a green arrow may appear alone or together with a red or green light. While it is on, the motorist's turn is protected. Oncoming traffic has been stopped and there will be no conflict for the turner. When the protected period ends, the green arrow disappears. If a full green remains lit, turns remain legal - but they are no longer protected. Turners must yield to oncoming vehicles.

The new installations are different. Green arrows don't just disappear; they change colors. For example, as westbound traffic on M Street approaches Wisconsin Avenue, it may encounter a green "through" arrow and a red right arrow. This means one may go straight ahead only. Left turns are prohibited entirely, and when the red right arrow is lit it prohibits right turns.

The new system is a bit more complicated than the old, to be sure. But if you're going to take a motor trip this summer, you'd better become accustomed to it. It's the latest thing in traffic control devices, and we'll be seeing more of it as time goes on.