A more detailed explanation of the S&P Price Channel Indicator with a complete 45-year history can be found here.
The price channel is currently 2418-3000, and the S&P 500 (SPY) closed January at 2823.81, closely chasing the over-priced level. Note that crossing into over-priced territory does not immediately signal a downfall. As the chart shows, the S&P 500 can spend 1-2 years in the over-priced region before the start of a bear market.
The price channel indicates whether the index is overpriced or underpriced with respect to (delayed) measurements of financial capital flowing into the economy. The price channel for the past 24 years is shown below.
Please note the actual value of the S&P 500 index is determined by market participants, and may often be outside the indicated channel. The Price Channel merely indicates the price levels that would be supported by financial capital which has been raised in the credit markets.