Palo Alto residents' ongoing frustration over worsening traffic surfaced again Wednesday night, even as city transportation planners tried to explain what they're doing to meet the public's rising demand for more accurate and informed traffic analyses.

Over the past several years, residents have expressed suspicions that city staff aren't adequately predicting how much added traffic will come with new developments when they are proposed. Either planners are using faulty numbers and analyses, or they are failing to take into account the cumulative traffic impact of numerous small developments, residents have asserted.

Just one member of the public spoke at the traffic study session Wednesday, but Planning and Transportation Commission members quickly picked up on the public's simmering discontent through their comments to staff.

Referring to a proposal by developer Jay Paul Co. to build an office complex on Page Mill Road near El Camino Real, commissioner Eduardo Martinez said that when the commission initially asked how bad the traffic delays were at that major intersection, "The response from you was, 'We don't know.'"

"It seems troubling that these studies seem a little oblique. They don't seem like they're coming forth with background information … that give us a level of comfort about what we're stepping into," Martinez said. "So I understand the fear of the neighborhoods of where this might be going. I don't think I've ever seen in my four years (on the commission) a project rejected on traffic issues; yet we see a problem that exists."

Commissioner Michael Alcheck said he appreciated residents' "skepticism," adding that it seemed that, when staff favor a proposed project, there could be an incentive "to downplay the impacts."

Alcheck also suggested the city review how well its traffic consultants have done in the past in predicting traffic problems, or the lack thereof. Commissioner Greg Tanaka also supported the idea.

The city's recently hired planning director, Hillary Gitelman, assured commissioners that her staff aims to provide the kind of informative and helpful analysis that the commissioners and the public are seeking.

In fact, transportation staff said Wednesday they have been working to develop a new model that will more accurately predict traffic problems brought about by new development. Working with Hexagon Transportation Consultants, the city now has data from 53 key Palo Alto intersections and 44 roadway segments, which will be used to create a baseline.

Currently, the city prepares an analysis of the potential traffic problems that each new development could cause, staff said. The scope of the report is determined by the number of new rush-hour car trips it could bring, with 100 or more triggering a full traffic-impact analysis, and fewer than 50 being documented through a memo, staff said.

Using Santa Clara Valley Transportation Authority guidelines for traffic-impact analysis, the reports assess how much traffic could be created by a development; the streets those added cars would use; and the impact on intersections in the area.

The analyses rely on several baselines, to take into consideration both the proposed development as well as other anticipated changes throughout the city and in neighboring cities: Existing traffic, which identifies current conditions; near-term, which adds up the cumulative traffic of all projects that have been planned or approved but not yet built; and future, which uses a forecasting model to estimate what traffic will be like in 2035.

By following the VTA methodology, the city's traffic models will take into account "growth anticipated locally and throughout the region," which is tracked by the VTA, Chief Transportation Official Jaime Rodriguez stated in a new report.

For Palo Alto specifically, the new forecast model also assumes the city will add 4,000 new homes and 25,000 jobs by 2035, according to the staff report. Those figures were based on actual growth between 1970 and 2010.

But the key measure of the impact of a development is the amount of delay at intersections.

At major intersections monitored by VTA, such as El Camino Real and Page Mill Road, delays of up to 80 seconds are considered acceptable. But if a new development's traffic increases the delay, the change is considered "significant," and the developer would be required to take steps to prevent the problem, Rodriguez's report states.

For most other Palo Alto intersections, those not monitored by the county, planners use a more conservative standard, the report notes. A delay at a signal of 55 seconds, and up to 35 seconds at a corner without a traffic light, is considered acceptable. But when newly created traffic causes longer delays, then the city will ensure the developer plans to stem the congestion.

The issue of cumulative impacts has come to a head in recent years, with critics of Palo Alto's plan to shrink California Avenue from four lanes to two charging that city planners did not take into consideration how future developments' traffic will clog the newly slimmed business boulevard.

At a presentation last week, developer Ray Paul of the Jay Paul Co. said the company's proposed office complex could bring 900 more car commuters to the site. At the meeting, residents who live near the property asked rhetorically how all those cars could get to and from work without causing congestion.