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If you haven't already done so, please read the Expanded version of Defect # 2.

Defining "Approach Speed"
- history, 1996 - 2006 -

Early History of the Table

From 1996 until 2002, the Yellow Change Interval table published in the main portion of the Traffic Manual was in increments of 5 kilometers per hour.  While the "metric" version of the table had been revised/obsoleted by a  memo issued April 1998 (
available online, in a Manual section called "New Policy"), until November 2002 the obsolete metric table remained in the main portion of the official electronic edition of the Manual, often confusing viewers who did not know it was necessary to check for revisions.
The obsolete ("metric") table is no longer available on the CalTrans website.  Here is a copy of it, from my files:

(for a larger copy, click on the image)

                    "Metric" Table

Click on image above for a larger copy of the obsolete ("metric") table.

In 2002 the California State Auditor wrote:

"From the survey, traffic engineers determine the maximum speed that 85 percent of the motorists are traveling - commonly referred to as the 85th percentile - and compare it to the posted speed.  Often a speed survey shows that most traffic is driving faster than the posted speed.  In these circumstances, the chief [ of the CalTrans office of Electrical Systems ] stated, traffic engineers should use the higher speed for determining the minimum yellow light interval."
(California State Auditor's report, page 40 - see Links page.)

The Auditor also recommended:

"To avoid the risk of legal challenges, local governments should petition CalTrans to clarify its traffic manual to explain when local governments should use either posted speeds or the results from speed surveys to establish yellow light time intervals at intersections equipped with red light cameras."
(California State Auditor's report, page 46 - see Links page.)

A 2003 Federal Highway Administration / Institute of Transportation Engineers "Informational Report" recommended:

"If available, the 85th percentile speed should be used as the approach speed in this equation.  In the absence of 85th percentile speed, some jurisdictions use posted speed as the approach speed.  In most cases, using the 85th percentile speed will produce intervals that are more conservative (i.e., longer).  In no case should the approach speed used in the calculation be less than the posted speed limit."
("Making  Intersections Safer:  A Toolbox of Engineering Countermeasures to Reduce Red-Light Running," FHA / ITE, 2003, Chap. 3.)

Action in 2004

From the minutes of the Aug. 12, 2004 meeting of the California Traffic Control Devices Committee.
Notes added by appear in double square brackets [[  ]].

8-12-04 Minutes.

04-B Yellow Change Intervals Timing for the Signals

Chairman Fisher [[ employee, City of LA ]] asked Committee Member Hamid Bahadori to address the agenda item of yellow change interval timing for signals.

Hamid Bahadori [[ employee, Auto Club of So. Cal ]] noted that the yellow change interval timing issue needs immediate attention and clear policy direction from the CTCDC, especially in light of the increasing number of automated red-light cameras being used to enforce red light violations at signalized intersections. Agencies are using different methodologies to determine the yellow timing. Since there is no definition for the approach speed in the Traffic Manual, or now in the California Supplement, the issue is what is the approach speed. Is it a posted speed limit or it is an 85-percentile speed? When a motorist receives a citation, the citation does not hold up in the court because approach speed has not been defined. The red light cameras are issuing tickets on a differential of one tenth of a second. People are challenging red light violations based on the lack of definition of approach speed. The law can be defended if there is a definition for the approach speed and minimum yellow timing for the left turn movements. Hamid referred to the State of Arizona DOT. They have a more detailed policy in regards to yellow timing for the through movement and for left turn movements. Hamid stated that the following two questions need to be addressed:

? Clarification on what is the approach speed.
? Should the same speed be used for all movements, if not, then does policy need to be established for the left turns approach?

The following Section of the California Vehicle Code (CVC) requires yellow timing at signalized intersections where such automated systems are used, to be established according to the Traffic Manual (now the California Supplement).

21455.7. (a) At an intersection at which there is an automated enforcement system in operation, the minimum yellow light change interval shall be established in accordance with the Traffic Manual of the Department of Transportation.

Gerry Meis [[ employee, CalTrans ]]stated that there is draft language in the agenda packet which addresses the approach speed issue. However, it does not address the left-turn movement yellow timing. This is only a draft, any suggestions and recommendations will be considered. Gerry agreed that due to red-light camera enforcement and that the CVC says the yellow interval shall be established in accordance with the Traffic Manual, this section needs improvement. He welcomed any suggestions for the improvement of the section. Gerry inquired, if a major change is recommenced to the Section, is there a need to change existing law?

Chairman Fisher stated that there is a need to clarify left-turn and right-turn yellow intervals. He also suggested flexibility so that a local agency can give more yellow time if it is needed. He added that the yellow interval should be based on the 85% percentile speed. The posted speed limit sometimes is not consistent with the 85 percentile due to political influence. If the posted speed is not consistent with the 85 percentile, then there will be a problem and more motorists may pass through the red light. It could also be a safety problem.

Hamid added that the Traffic Manual allows the use of the posted speed limit to determine the location of advance loops at signalized intersections.

Chairman Fisher invited the audience to give input.

Ahmad Rastegarpour, Caltrans Headquarters Office of ITS Development & Support, stated he believes that in determining the minimum yellow timing for a signalized intersection, appropriate judgment must be exercised. That judgment would be based upon numerous factors, including the posted speed, the 85th percentile speed (if available), the intersection geometrics, the traffic volumes, through and left turn movements, and other factors that would not be readily apparent to motorists using the intersection. To determine the yellow timing for the left-turn pocket, there is no guidance available. Therefore, a field review is needed to determine the left turns yellow timing which will be based on geometrics, left-turn pocket, the number of left turn lanes and other factors. He does not recommend a minimum yellow timing be set which would prohibit practitioners from adjustment based on field conditions.

Hamid commented that he was also not in favor of putting limitations on the flexibility for practitioners. However, since the red-light enforcement, vendors may be manipulating yellow timing to issue more red light violations. And since the approach speed is not defined, they are using their own criteria for the yellow timing. Secondly, the courts are throwing violations out because the approach speed not being defined.

Gerry Meis stated that the Department would consider all suggestions from the Committee and work with the Committee to resolve this issue. He said he was not in favor of going back to the legislature to amend the law. Marianne Milligan, City Attorney’s Office of Costa Mesa, stated that when a City adopts a document by reference such as the Caltrans Traffic Manual, the City also adopts later amendments. She commented that she does not believe that changes in the Caltrans Traffic Manual would require action by the legislature.

George Allen [[ employee, City of Garden Grove ]] commented that for years their jurisdiction used 3 seconds for the left-turn lane and 4 seconds for the through movements. Since the introduction of red-light cameras, the 85-percentile speed has been used to determine the yellow timing. The 85-percentile speed was measured between the advance loop and the limit line. The 85- percentile speed was used to determine the yellow timing for the left-turn and the through traffic. In his opinion, there should be 3.2 seconds minimum for the left turns and use the table for the through movement. George added that there is a need to define the approach speed. If all agencies are using the same definition then everyone is consistent.

There was a lengthy discussion regarding the approach speed definition and the minimum yellow timing for left turns.

Hamid cited a example where if a roadway has an approach speed of 40 mph, and an agency uses a 3 second minimum yellow timing for the left turn, then a citation will not hold up in court, because there is no separate guideline for the determination of yellow timing for the left-turns. The Traffic Manual says that the minimum yellow timing is based on the approach speed, so it applies to both through and left turn movements.

Marianne stated that local agencies could defend their case if the definition of approach speed is clear. It could be an 85 percentile or posted speed limit. Approach speed must be clarified. If the Committee suggests that the left turn must use a minimum of 3.2 seconds, then all agencies will be consistent, especially when red-light cameras are used.

Farhad Mansourian suggested that this is a discussion item and regardless of what is done here today, the Committee cannot take action. He suggested forming a subcommittee to work on this and bring draft language for the Committee’s consideration during the next meeting.

Chairman Fisher established a subcommittee chaired by Hamid Bahadori [[ employee, Auto Club of So. Cal. ]] with the members being Farhad Mansourian [[ employee, County of Marin ]] , Mark Greenwood [[ employee, City of Palm Desert ]] and Gerry Meis from the CTCDC. Two outside members, Marianne Milligan, City of Costa Mesa and George Allen, City of Garden Grove, were asked to provide input. The subcommittee was asked to develop draft language for the next CTCDC meeting.

The subcommittee wrote a report which was heard at the Committee's Dec. 8, 2004 meeting.  A copy of the report was included in the
for that meeting, at pages 6 - 18.

The minutes of the Dec. 8 meeting became available on the Committee's site, on February 2, 2005.   I have been told that the Committee debated the matter for three hours.  The final vote was 6 - 2 (noes:  the CHP and CalTrans representatives) to adopt the approach reflected in the Draft, below.  Four of the "yes" votes came from  representatives from two cities  having red light cameras, and two representatives from the northern and southern California AAA automobile clubs.  There was also a vote on a motion to set the minimum for a left-turn yellow at 3.0 seconds.  That vote initially was 5 - 2, then 7 - 0.  The Dec. 8 minutes indicate that Mr. Meis of CalTrans was one of the (initial) "no" votes, but do not reflect who the other was.  I made an inquiry to Committee staff, which said that the other "no" was CHP Capt. Duncan, and that Mr. Lem  [[ employee, Cal. State Auto. Assn., AAA ]] had left and did not vote.  The minutes also noted that a minimum of six votes were required for passage of a motion - meaning that if some of the reps from the auto clubs and the red light cities had been removed because of conflict of interest, neither motion could have passed!

On Friday Dec. 17 the following Draft was emailed to Committee members, along with a request that they submit their comments by Dec. 21.  That deadline was later extended to Dec. 27.


Draft For Discussion Only

Section 4D.10 Yellow Change and red Clearance Intervals


The purpose of the yellow signal indication is to warn traffic approaching a traffic signal that the related green movement is ending or that a red indication will be exhibited immediately thereafter and traffic will be required to stop when the red signal is exhibited.


The minimum yellow light change interval shall be in accordance with Table 4D-102.  The posted speed limit, or the prima facie speed limit established by the California Vehicle Code (CVC) shall be used for determination of the minimum yellow change interval for the through traffic movement.

The minimum yellow light change interval for a protected left-turn/right-turn phase shall be 3.0 seconds.


The minimum yellow light change interval time for the through movement and the protected left-turn/right-turn may be increased based on a field review or by using appropriate judgment.  That judgment may be based on numerous factors, including, but not limited to, posted speed, 85th percentile speed, intersection geometry and traffic volumes.

Table 4D-102 Minimum Yellow Light Change Interval









25 or less

40 or less



























Jan. 2005 Official Action Eliminates Use of Term "Approach Speed"

On Jan. 26, 2005 the Committee officially changed the "yellow interval" table (see Defect # 2 on the Home page) to refer to Posted Speed instead of Approach Speed.   Their decision to use Posted Speed was at odds with both the CVC 21455.7 reference to "designated approach speeds" as well as with the Federal Highway Administration recommendations.
See for the new table, and some background.

The change was published on Jan. 26, 2005, at .
The published Policy Directive contained a recommendation for a review after one year, so I sent out the  following letter.


From (

To Mr. Will Kempton, Director, CalTrans, (916) 654-5266 fax (916) 654-6608

Dear Director Kempton:

A year ago the California Traffic Control Devices Committee discussed, and later adopted, a policy that effectively shortened the minimum length of yellow time on traffic signals equipped with red light cameras.

That policy, later published as Traffic Operations Policy Directive 05-01 of January 26, 2005, included a recommendation by the Committee that there be a review after a year.  That year now being about up, I expect that the policy will be coming under Committee consideration again, and would like to make the following observation and request.

Per the minutes of the December 8, 2004 meeting of the Committee, there were two votes on the policy.  The first vote was 6 - 2, to adopt policy applicable to the length of yellows for straight-through movements only.  In that vote, four of the aye votes came from Committee members who in my opinion had a conflict of interest.  Two of those four were employees of cities that operate red light cameras, and another two were employed by auto clubs whose main revenue stream comes from the sales of car insurance.  Had any one of the four voted differently, the policy would not have passed, as six ayes are required for passage of a motion. 

The second vote, also during the December 8 meeting, was 7 - 0 to adopt policy applicable to the length of yellows for turning movements.  Three of the seven ayes came from members who in my opinion had a conflict of interest.  Had any two of the three voted differently, the policy would not have passed.  Adding to the appearance of impropriety, during the consideration of that policy the committee requested legal advice from an attorney who was there representing a city which operates red light cameras.   

I would suggest that when the policy comes under new review, that Committee members from cities or companies which will be directly affected financially be excluded from the discussion and the voting, and further, that the Committee obtain its legal advice from a neutral source.



Here is CalTrans' reply of Dec. 8, signed by Gerry Meis, Chief, Office of Signs, Markings and Permits:

The One-Year Review

At its Feb. 23, 2006 meeting in Sacramento, the CTCDC was scheduled to re-evaluate the yellow timing.
(The agenda for the meeting is available on the CTCDC's site - see link above.)

The meeting did not go well.  The committee voted to not even have a discussion of changing last year's decision !

The details:

The meeting was on the 23rd, at a CHP building in West Sacramento. 

The item came up just before noon.  The chair announced that the only decision in front of the panel was whether the matter would be studied and brought back at a later meeting.  (The item was listed on the agenda -  - as "Discussion item - Re-evaluation of the decision regarding the yellow light timing."  - and Gerry Meis' short report - at page 28 of the agenda - indicated that during the meeting there would be a discussion of the length of the yellows.  Had CalTrans rep Meis been present - he was ill - that discussion might have occurred.)  
The chairman also announced that no public input would be taken, since the item was listed as a discussion item only.   This would have had the effect of preventing public attendees from addressing the committee at all, even on the issue of whether the item should be studied.  He made that announcement just before adjournment for lunch.  However, after lunch the chair said that he had changed his mind and would allow the public to speak. 
All it would have taken was for ONE of the seven members in attendance to move to have the matter studied and brought back.  Not one of them did so.  So, unless CalTrans decides not to adopt the CTCDC decision to not revisit the issue (after all, it is just an advisory committee), yellows in CA will continue to be based on the posted limit, and with 3.0 seconds minimums on left and rights.

After the meeting, a public attendee wrote a letter to Meis, and it is reproduced below, with the author's permission.


You missed the vote on opening up for review signal timing standards. The committee voted to not review current practice. Of course, the two knowledgeable members of the committee who voted against this last year were not there - CHP and Caltrans.

Regardless of the CTCDC, you are the final safeguard to assure the lives of over 30 million and the million of visitors to the state are protected.

Best practice are those practices that have been proven safe. The current practice has been proven to be inadequate, with negative consequences for thousands; some die and many more are injured each year as a result.

Members on the committee took positions to protect their interest, not the peoples. As one of strongest proponents of not opening it up for review said to me privately, we have too many intersections to do reviews. The same argument he made to eliminate the requirement to assure sight distances at intersections and driveways. Where the majority of the most serious accidents occur. Again, taking a position against remedies because you don't want to do them, is not exactly championing safe roads.

Someone on the committee has to represent more than their agency's view, but the peoples best interests', too.

That leaves you, because as the state traffic engineer, it is ultimately laid at your doorstep.

Please do the right thing. The current practice is not a standard at all, nor does it contain any checks against deviant practices either. The 1988 MUTCD section 4b-20 is still the clearest possible language to assure that if a defect is discovered, that remedies must be applied. Turning quantified unsafe condition, unaltered, into a profit center for a city... is not a remedy!

The overriding safety emphasis of the controlling federal traffic safety law is that "Signal Operations Must Relate to Traffic Flow" (MUTCD 4B-20). The express provisions of the MUTCD to accomplish this mandate require an engineering study as a precondition to determining the proper phasing and timing of a traffic control signal, and it shall be documented. The timing of traffic signals is determined by an engineering study. The engineer can either chart how long it takes traffic to stop after the onset of yellow and set the signal to meet that need, or use the ITE kinematic formula; approach velocity (vehicle speed) is an essential part of that formula; traffic volume is not a factor in the formula. Either way, these are only the starting point of the process. The timing is to be further adjusted to assure the needs of the traffic are met.

1988 MUTCD Section 4B-20: Signal Operation Must Relate to Traffic Flow

“Traffic control signals shall be operated in a manner consistent with traffic requirements. Data from engineering studies shall be used to determine the proper phasing and timing for a signal. Since traffic flows and patterns change, it is necessary that the engineering data be updated and re-evaluated regularly.”


Chad Dornsife, Director
Best Highway Safety Practices Institute
PMB 193
25 NW 23rd Place, Suite 6
Portland, OR 97210-5599

For the more recent history of the yellow light length, see the Expanded version of Defect # 2.

If you need information from CalTrans, call CalTrans headquarters at (916) 654-5266.